Using Arrows and Circles in pictures
I use the app Skitch for annotating my pictures. Here's the link to it:
Skitch - Snap. Mark Up. Send. by Evernote
It's free to download and you can add different annotations like arrows, circles and text in different colours to help explain things better.
You can also do things like pixelate, for example, and add smiley faces
If you have other tools which you use to annotate, feel free to share!