I can't count the number of ways you could use Skitch in teaching!
Skitch is part of the Evernote family, which I'm sure you've gathered by now I'm pretty fond of (being a Yorkshire man, have I told you it's free?). Its a cool app that you can use for, annotating images like this.
It's recently been updated for the iPad so it now includes capturing web pages and being able to annotate them along with annotating normal photos, drawing on maps or just a blank canvas for sketching.
Now if you don't have an iPad you can get the software on most platforms ( and it's still free) there is a windows desktop download along with iOS. Unfortunately my iPad is only on loan so it will have to be returned eventually.
Anyway what about using it in teaching? Add pictures to your moodle page that have been annotated to give the students information, for example the size of nuts or bolts to help them to pick the right spanner first time or get organised for doing a job.
Take a picture of Health and Safety issues that you notice to make them aware of later on in class, making sure there is no risk at the time. (clarify)