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:
- The mood/emotion (from colors and expression)
- The general subject (from composition)
- 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
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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