Aside from its library of readymade stamps, iAnnotate also lets you create and save custom stamps. Tap a finished ink annotation (such as one made by Pencil or free-hand highlighting), and then select Stamp from the Annotation Ribbon in the upper-right. After naming the stamp, it is added to your library for future use.

Creating a custom stamp from an ink annotation

Also, in the stamp library, you can tap the plus sign in the upper-right to bring up a menu that allows you create a custom stamp from your iPad photo library or camera.