For me the minimum size 768 x 768 pixels is OK !!
That would be a ridiculously small image then! The standard frame of every ship photo page here has a horizontal size 800p, then you click on "Full Screen" & you get a disappointment that the actual photo is even less than what you initially saw.
I still say, that as long as the bigger side
of the original photo is not less than 1024p it should be Ok. Of course other standards are applied too, cause it might be small & not sharp pic at the same time.