ComicStrip is a nice app (.99) for the iPad/iPhone. This allows students to create a digital story as they would w/ any online comic strip creator. A user can upload or take a photo, edit their layout, and then add speech bubbles and stickers. While it is not the most robust product around it does offer a nice and inexpensive way to explore this 21st century skill.
I recommend checking out ComicStrip by clicking here.