handle big files (resize, instead of reject, up to a limit of course)

we should be able to accept much bigger files nowadays (tbd what's the reasonable size but flagship smartphones can produce huge photos)
we can accept them (up to reasonable limit) and then save resized / compressed, to a I guess maximum 4096x4096?
after all we are not a general purpose image storage service, but the idea is just to support bloggers on a best effort basis.

probably different rules should be for photos that are just proxied, i.e. use case when a photo editor makes awesome high resolution photos, stores them on their own hosting service at super high resolution, then we should allow them to be way bigger and not try to compress too much? rationale behind is that externally hosted photos wont eat up our space in a long run, if they do, we can always remove high resolution versions and fallback to smaller ones? if source material is there, it will rebuild, if not...then there will be a "regular" resolution version