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I believe YT is 2MB

YT is a videosite … the posterframes or thumbnails are not the mainfocus there …

Good question! I can't remember ever having seen such a message anywhere except on special photography websites where it is also about completely different orders of magnitude. I would say a max of 5mb should be enough. The photos I generally upload are 1920x1080px jpgs (100%), depending on the image content between 2 and 4MB in size. That‘s generally a good enough quality to be compressed again elsewhere beyond my control.

X/Twitter Maximum file size: Up to 5MB for photos and GIFs on mobile. Up to 15MB on the web.

Different sources say different things for FB, but i found a max of 30MB on a few more recent sites.

LinkedIn 8MB

Pinterest 20MB

YouTube Banner image 6MB

Snapchat 5MB

… just a quick research 🖖🏼👁🤙🏼