The picture below will be the Skitch markup with parts of the frog labeled.
So, very quickly, you can have a picture labeled by the students to illustrate the parts they know, or show the parts they don't. Skitch allows for easy annotation of a picture or document, as well as easy saving of the new and marked up diagram.
Another way Skitch could be used is by having your students take pictures of an experiment to document the process. They could open the pictures in Skitch to label and document the what the picture is showing. Once all of the pictures are marked up and completed, the students could use them in some other program to report out their findings - like a word processor or presentation software. Skitch would allow the student to explain the picture in detail, right on the picture.