I need to store profile pictures on our servers. The profile pictures will be used on the web and mobile devices. The largest use is 400 x 400 pixels. On high res devices that's 800 x800. Should I assume that resolution of future devices will increase and store larger images? Do I really need 800 x 800 for a 400 x 400 image?
Because you will be sending these images over the web you should strongly consider storing multiple versions of the image in different sizes (i.e., 32 x 32, 200 x 200, 400 x 400, 800 x 800). This is for several reasons:
It's worth noting that in the web world this approach is gaining steam (look up "responsive images").