Technique: Moving full-res images back to your iPhone
One of the “rules” of iPhoneography (if you’re a purist) is that all image manipulation — filters, effects, cropping, etc. — be done on the iPhone. The default setting for an iPhoto sync is to remove all images from the camera roll after they are synced to your computer. Adding an image back to your iPhone by syncing to an album reduces the image size to no larger than 640 x 480 pixels.
If you want work on the full-size image that has been synced and removed from your iPhone, simply email the full-size image to one of the email accounts that you receive on your iPhone. From Mail, you can save the full size image back to your camera roll where you can edit the image with the iPhone apps of your choice, as if you’d just taken the photo.