When I was thinking of what app or feature that I would like to share, I decided to look at what I use on a regular basis, and what a teacher or student could use on a regular basis as well.
The app I use most frequently when designing training materials is Skitch. Skitch is a free application both for use on a computer and also iOS/android device. Skitch allows you to edit images and draw circles, highlight, draw arrows, smear names, enlarge/shrink around the images then drag them into a working document. If you open an account through Evernote, you can get your images anywhere you log into your Evernote account. That feature is AWESOME because you can get them from any device.
When I downloaded the app to my iPad, I liked it even better!! It is so easy to use. It is simple to drag your edited image to your project. Simply grab the bottom and drag it to the document you wish.
Here is a sample of my gallery when I open the Skitch app. As you can see, there are a lot of images that I have edited for training purposes. I can open one of those images or create a screen snap from start and edit that. I can also edit an image that is saved on my device.
Give it a try! Download the app by going to http://evernote.com/skitch/.