I've tried jpeg and png format so far.  Images are right at 2MB and 4k x 4K pixels.  Well below the restrictions.  Any clue why I can't get the picture to accept?
 
 
image1.png (remove)
Image selected cannot be uploaded because it does not meet the listed requirements. Please remove first!

I had an issue once where it didn't like the browser I was trying it from too. 
Switched to a different one & worked fine ...