I use a variety of tools to manipulate photos that I take. Especially when I want to consolidate photos together for blog posts or displays. One of the tools I like to use to do this is Pic Collage.
It is incredibly easy to use. You tap the screen to add photos, change background or add stickers (which can be purchased in app). You can rotate pictures using 2 fingers, and tapping a picture brings it to the foreground. Finished collages can be shared via email, standard social networking or just saved to photo library.
Here is an example of a photo collage (extra marks for working out where I took these photos!)