What are some good photo editing apps that are free for the iPhone?

Alright so can someone tell me what are some good editing apps that you use or that you have found?
Is there a app where I can make a photo where half of it is one picture and the other half is another picture? I was going to edit a photo for my friend {half of it being half of his face and the other half, half of the celebrity he likes face (Andy Biersack) Any suggestions?

Yes, the App Store has good number of photo editing and effects apps.
Since you need to include more than one photo, try using collage apps like PicCollage, Diptic etc.