Install the belt CLI skill: npx skills add belt-sh/cli

YouTube Thumbnail Design

Create high-CTR YouTube thumbnails with AI image generation via inference.sh CLI.

Quick Start

Requires inference.sh CLI (belt). Install instructions

belt login

# Generate a thumbnail
belt app run falai/flux-dev-lora --input '{
  "prompt": "YouTube thumbnail style, close-up of a person with surprised excited expression looking at a glowing laptop screen, vibrant blue and orange color scheme, dramatic studio lighting, shallow depth of field, high contrast, cinematic",
  "width": 1280,
  "height": 720
}'

Specifications

Spec	Value
Dimensions	1280 x 720 px (minimum)
Recommended	1920 x 1080 px
Aspect ratio	16:9
Max file size	2 MB
Formats	JPG, GIF, PNG

The 120px Test

Your thumbnail appears at roughly 120px wide on mobile — that's how most viewers first see it.

At 120px, viewers must be able to identify:

1. The mood/emotion (from colors and expression)
2. The general subject (from composition)
3. The text (if any — only if large enough)

Test: view your thumbnail at 120px width. If it's a muddy blur, redesign.

Safe Zones

┌─────────────────────────────────────────────┐
│                                             │
│   ✅ SAFE FOR TEXT AND KEY ELEMENTS         │
│                                             │
│                                             │
│                                             │
│                                             │
│                                       ┌───┐ │
│                                       │ ⏱ │ │ ← Timestamp overlay
│                              ┌────────┴───┘ │    (bottom-right)
│   ┌────┐                     │  DURATION    │
│   │ CH │ Chapter marker      └──────────────│
└───┴────┴────────────────────────────────────┘
     ↑ Bottom-left: chapter/progress markers

Avoid placing critical elements in:

• Bottom-right corner (video duration timestamp)
• Bottom-left corner (chapter markers, progress bar)
• Extreme edges (cropping varies by device)

Color Strategy

High-Contrast Pairs That Work

Combination	Mood	Best For
Yellow + Black	Urgency, attention	Tech, business, lists
Red + White	Energy, excitement	Entertainment, reactions
Blue + Orange	Professional contrast	Education, tutorials
Green + White	Growth, money	Finance, success stories
Purple + Yellow	Premium, creative	Design, art, creativity
White + Dark	Clean, minimal	Luxury, minimalist channels

Color Rules

• Background and text/subject should be complementary or high-contrast
• Avoid same-temperature colors touching (red on orange = mud)
• Use 3 colors maximum per thumbnail
• Saturate more than real life — thumbnails compete with bright UI

Text on Thumbnails

When to Use Text

• Lists/numbers: "7 Tips", "Top 10"
• Strong opinions: "STOP Doing This"
• Results: "$10K in 30 Days"
• Comparisons: "vs" between two things

When NOT to Use Text

• The video title already says it (redundant)
• The emotion/visual tells the story
• You can't make it large enough to read at 120px

Text Rules

Rule	Reason
Max 6 words	Readability at thumbnail size
Min 60pt equivalent	Must be legible at 120px width
Bold sans-serif font	Thin fonts disappear at small sizes
Contrast stroke/shadow	Ensures readability on any background
No small text	If it's not readable small, cut it

Face Expression Psychology

Thumbnails with faces get higher CTR than faceless thumbnails. Expression matters:

Expression	CTR Impact	Best For
Surprise/shock	Highest	Reaction, reveal, discovery content
Curiosity	High	Tutorial, how-to, tips
Excitement	High	Unboxing, reviews, announcements
Concern/worry	Medium-high	Warning, mistake, problem content
Confidence	Medium	Expert advice, authority content
Neutral	Lowest	Avoid unless your brand is minimalist

Face Composition Rules

• Face should fill 30-50% of the thumbnail
• Eyes looking toward the text or subject (directs viewer attention)
• Eyes looking at camera = connection. Eyes looking at object = curiosity.
• Place face on one side (usually left), text or subject on the other

# Generate a face-forward thumbnail
belt app run falai/flux-dev-lora --input '{
  "prompt": "close-up portrait of a man with genuinely surprised expression, mouth slightly open, raised eyebrows, looking at camera, left side of frame, vibrant teal background, dramatic rim lighting, YouTube thumbnail style, high contrast, cinematic",
  "width": 1280,
  "height": 720
}'

# Generate a face-looking-at-subject thumbnail
belt app run bytedance/seedream-4-5 --input '{
  "prompt": "person looking amazed at a glowing holographic chart showing upward growth, dramatic blue and green lighting, right side profile view, dark background, tech aesthetic, high energy",
  "size": "2K"
}'

Thumbnail Patterns by Content Type

Tutorial / How-To

belt app run falai/flux-dev-lora --input '{
  "prompt": "overhead flat lay of organized workspace with laptop showing code editor, colorful sticky notes, coffee cup, clean bright background, professional setup, tutorial style composition, warm lighting",
  "width": 1280,
  "height": 720
}'

Before/After

belt app run falai/flux-dev-lora --input '{
  "prompt": "split composition, left side dark and messy disorganized desk, right side bright clean organized minimalist workspace, dramatic contrast between chaos and order, clear dividing line in center, high contrast",
  "width": 1280,
  "height": 720
}'

Product Review / Comparison

belt app run falai/flux-dev-lora --input '{
  "prompt": "two products facing each other with dramatic lighting and sparks between them, competition battle concept, dark background with colorful rim lighting, versus comparison style, high energy, product photography",
  "width": 1280,
  "height": 720
}'

Listicle / Number

belt app run falai/flux-dev-lora --input '{
  "prompt": "dynamic arrangement of 7 different colorful objects floating in space against dark gradient background, each item distinct and clearly separated, energetic composition, vibrant saturated colors, studio lighting",
  "width": 1280,
  "height": 720
}'

A/B Testing

Test one variable at a time:

Variable	Test A vs B
Face vs No face	Same composition, with/without person
Expression	Surprise vs curiosity
Color scheme	Warm vs cool palette
Text vs No text	With/without text overlay
Background	Bright vs dark
Composition	Left-facing vs right-facing subject

# Generate variant A
belt app run falai/flux-dev-lora --input '{
  "prompt": "..., bright yellow background, ...",
  "width": 1280, "height": 720
}' --no-wait

# Generate variant B (same prompt, different background)
belt app run falai/flux-dev-lora --input '{
  "prompt": "..., dark navy background, ...",
  "width": 1280, "height": 720
}' --no-wait

Thumbnail Checklist

• 1280x720 minimum (1920x1080 preferred)
• Under 2MB file size
• Passes the 120px squint test
• No critical elements in bottom-right (timestamp) or bottom-left (chapter)
• Max 3 colors, high contrast
• Text (if any) is max 6 words, bold, with contrast stroke
• Face expression matches content energy (if applicable)
• Doesn't duplicate the video title
• Stands out from surrounding thumbnails (check your niche)
• Works on both light and dark YouTube backgrounds

Common Mistakes

Mistake	Problem	Fix
Too much text	Unreadable at thumbnail size	Max 6 words or no text
Low contrast	Disappears in the feed	Use complementary colors
Cluttered composition	Eye doesn't know where to look	One focal point
Generic stock photo feel	No personality, gets skipped	Authentic expressions, unique angles
Tiny details	Lost at 120px	Bold, simple shapes
Same style every video	Viewer fatigue	Vary within brand guidelines
Misleading thumbnail	Kills trust, hurts retention	Match the actual content

Related Skills

npx skills add inference-sh/skills@ai-image-generation
npx skills add inference-sh/skills@image-upscaling
npx skills add inference-sh/skills@prompt-engineering

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