You edit your photos all the time before you post them on Facebook or sending them to your friends. If you use iOS photos app to edit or crop photos, it overwrites the original photo. How can you keep your original photo without sending the photo to yourself via email etc?
If you edit photos like removing red eye, cropping, auto enhancing or just rotating and save the photo, You will lose original photo. It is very easy to keep original photo. This does not involve silly tips like send original photo to yourself or take a screenshot of your original photo to duplicate or copy the photo.
How to edit photo & keep original photo on iOS/iPhone 5/iPhone 4S/iPhone 4/iPad
1. You need to have Photo Stream enabled on your iPhone or iPad. This gets enabled by default If you add iCloud account. If you turned it off, You can enable it by going to Settings > iCloud > Photo Stream > Turn on My Photo Stream.

You can use Photo Stream If you have iOS 5.1 or later on your iPhone, iPad, iPhone Touch.
2. To edit a photo, Go to Photos app > Photo Stream (at the bottom) > Select photo you want to edit > Edit > Save > Tap “Save to Camera Roll” to save edited version of the photo.

Your Camera Roll still contains original photo. Thats it. This way, You can edit photos without worrying about keeping actual photos and post them to Facebook or send them via email.
Also read, How to edit/upload videos to YouTube on iPhone/iPad