I guess that pixel dimension requirement is very strict, aka it does not allow image to have dimension flipped, aka no tall images are allowed.
As image I tried to upload is 864x1617 pixels, it does not adhere to strict restriction of 1920x1080 pixel restriction.
That is a problem for many photos/screenshots taken on mobile, as dimensions are often flipped.
Mods,
I wonder is that can be changed in future to 1920x1080 OR 1080x1920 or something in that fashion?