The required image size for square thumbnails must be 1080x1080px, and the maximum file size is 500KB. The required image size for vertical thumbnails must be 1080x1920px, and the maximum file size is 700KB. Furthermore, the specific format of the thumbnail needs to be in .JPG, .JPEG, or .PNG
If you are struggling to match the required image size and the maximum file size, we recommend exporting your thumbnail as JPG, or looking into an image compression tool.