iPhone photos are low resolution (640x480)
Have you shot any of those images with the "other" camera? The "face time" camera will only shoot a resolution of 640x480 and the "regular" camera shoots the higher resolution (5MP). Apple specs of the iPhone 4 states the face time camera shoots VGA (see http://support.apple.com/kb/sp587).

The high res camera shoots frontward. The face time camera shoots backwards (it is located near the ear speaker in the top of the phone above the screen). When using the camera app, if you press the icon that has the circular arrows, the camera changes to the one facing you (you will see yourself in the screen). That is the "face time" camera and shoots low res.