IIRC, you can
(used to) resize an iPhone image by emailing it to yourself and
(I believe) you can resample/resize the image from the stock software.
I am sure there are paid apps out there for it, but Apple doesn't
(didn't) allow you to do that natively. Android phones can. I only casually use iOS, maybe someone out there can definitively tell us if what used to work, still does.
[edit] not gonna lie, i'm more than a little interested in what you're looking for.