How to check a file
Open the checker and drop in one or more JPG, PNG, or WebP files. The specifications panel opens by default on this page. For each candidate it reports exact pixel dimensions, whether the shape is 16:9, and whether the file is below the 2 MB preset. Green and red marks are independent: a 1920×1080 image has the right shape but does not exactly match the 1280×720 preset. A square 800×800 image fails both the exact preset and landscape ratio.
Use these results as an export checklist. If your image is too heavy for your intended workflow, try exporting a JPG with modest compression and inspect the small preview again. If your image is square, cropping to landscape is usually more deliberate than stretching it, because stretching distorts faces and text. Keep a copy of the original so you can move the crop.
Understanding the 2 MB marker
The red file-size state means only that the image exceeds this tool's conservative preset. It does not claim that every YouTube upload must be under 2 MB. YouTube's current help page lists a 2 MB limit for video thumbnails uploaded on mobile and a larger limit for desktop uploads. Its current recommended video-thumbnail resolution is 3840×2160, with a minimum width of 640 pixels.
That distinction matters when a creator exports a 3 MB file from a desktop design tool. The checker should still mark it red for the requested 2 MB preset, while its explanatory text should direct the creator to the official device rules. The browser preview can still display the image and evaluate its design. File eligibility and visual clarity are different questions.
Design checks after the dimensions pass
A technically neat export can still perform poorly as a picture. Look at the 168×94 sample for words that collapse into strokes. Look at the lower-right guide for a price, face, or logo caught under the duration badge. A small, high-contrast subject usually survives reduction better than several equal-size subjects spread across the frame.
Try two exports of the same design: one with small decorative detail and one simplified. Keep the title and channel text identical while comparing them. The mock feed uses approximate card widths, so use it to catch obvious failures and verify the final result in YouTube Studio as well. Nothing is uploaded when you check local files here. If the red mark comes only from the exact-size preset, read the aspect result before editing again. A correctly shaped larger landscape file may be useful for a different publishing workflow.
Official references: custom thumbnail guidance, A/B testing, and video title settings.