What is the optimal resolution/pixel ratio/file size for image upload to photo contest?

I recently ran a small test poll, and the participants reported that some of the images wouldn't render even thumbnails on mobiles. This could be image file size or dimensions, but I assumed the thumbnail would be automatically created to the correct size.

Hi Keith,
We had an issue two days ago, where the thumbnails did not render.
Can you send me the link to your PollUnit to philipp@pollunit.com?
Then I can have a look if there is still a problem.
Kind regards
Philipp