The official Lumina Client logo (stylized "L" or full text) must be legible. Do not place it in the corner. Center it in the upper third of the thumbnail.

: Avoid any imagery that violates community guidelines, such as violent or sexual content, to ensure your thumbnail isn't restricted. Minecraft client features to highlight in your thumbnail for version 1.20.1? Changing and Uploading Thumbnail Images - YuJa Support

for higher quality before compression, but keep the file size under Brand Consistency