The Camera Roll is the only space on your iPhone capable of holding full size images that other apps have access too. It's not convenient to get them there, especially if you have multiple images, but it can be done. If you're viewing a large image on your iPhone in either Mail or in Safari, you can tap it and save it to your Camera Roll. (iTunes modifies all photos in the folder to be screen size to save space.)
Note that most photo processing apps on the iPhone shrink images in processing. So don't expect them to stay full size if you start tampering with them.
Getting Full Size Images Onto iPhone
