TL;DR Start with the official rules (size, formats, limits, and policy). Then use a repeatable “promise + proof + curiosity” structure so your thumbnail sells the click without crossing into misleading territory.
Aspect ratio: try 16:9 (most used in players and previews).
For vertical videos: YouTube notes that 16:9 custom thumbnails can be replaced by an auto-generated 4:5 thumbnail on Home/Explore/Subscriptions (your custom can still appear on other surfaces).
Shorts: YouTube notes you can’t upload a custom thumbnail for Shorts like long-form; you can select a frame during upload, and can’t change it after.
From YouTube’s spam/deceptive policies:
“Misleading Metadata or Thumbnails” includes using titles/thumbnails/descriptions to trick viewers into believing content is something it is not.
2 Instavar’s thumbnail hook system
Think of a thumbnail as a 1-second pitch:
Promise: what do I get?
Proof: why should I believe you?
Curiosity: what’s the twist that makes me click?
If you can’t communicate those three, the thumbnail is decoration—not a hook.
3 Templates (safe by design)
Use these as starting points for the text overlay (keep it short):
“Stop doing X”
“Do X instead”
“I tested X for 7 days”
“The one setting that fixed it”
“Why X isn’t working”
“This looks wrong… but it works”
“3 mistakes killing your X”
“Before / After”
“From 0 → 1 (fast)”
“Copy this”
“I wish I knew this earlier”
“The real reason”
“The simplest way to X”
“You don’t need X”
“Watch this before you buy”
Guardrail: If the thumbnail makes a promise, make sure the video pays it off.
4 Quick QA checklist
Exports match the official spec (size, formats, file size).
Text is readable at small sizes (mobile-first).
Promise is accurate (avoid misleading packaging).
For Shorts, pick a first frame you’d be proud to ship as the “thumbnail”.
5 Official references
YouTube Help — Add video thumbnails on YouTube (specs + Shorts note): https://support.google.com/youtube/answer/72431?hl=en&co=GENIE.Platform%3DDesktop