What the checker tests
Checks against YouTube's published requirements, plus the problems that only show up once your thumbnail is live.
Resolution
Compares your dimensions with the 640px minimum, the long-time 1280 × 720 standard, and the 3840 × 2160 size YouTube now recommends.
Aspect ratio
Flags anything that isn't 16:9 and names common mismatches like 4:3 or 1:1, which show up with black bars or get cropped.
File size
Tells you whether the file fits under 2 MB for the mobile app and 50 MB for YouTube Studio on desktop.
Format
Confirms JPG or PNG, and warns about WebP, GIF, and other formats worth converting before you upload.
Timestamp corner
Looks for detail in the bottom-right corner, where YouTube places the video length over every thumbnail.
Transparency
Catches PNGs with see-through areas, which YouTube fills with a solid color you didn't choose.
