(To view a larger image with more detailed description tap or click its thumbnail.)
The screen shots above illustrate use of the Adobe Ideas application which supports many of the same features you would find in a full featured image editor. The use of the layers feature must be purchased, but as the shots above illustrate, there are many options available for free.
Verdict: Get Adobe Ideas for the iPad and iPhone here.
Disclaimer: I am not associated or employed by any company producing software reviewed on this site.