It appears there are a few file types Apple does not allow an application to register as a handler of - JPEG and PNG are on that list. You can define a custom file type/extension and register that easily enough (as in the link you referenced), but Apple seems to prevent applications from showing up in the action sheet to receive images. If you want to see what your info.plist should look like if it were possible to register as a handler of images, take a look at the Evernote app (it's free). They register as a handler of a variety of file types, including images and PDF. For PDF's, Evernote shows up as a target to send the file to, but never in images.
If I were to guess, I'd say this is because Apple doesn't want dozens of applications to show up as the target for an image the user wants to save in Mail, Safari, etc. If any app could register to receive images, it could get confusing for users as to where their saved photos went.