The week's Appy-Hour pick is an absolute essential for teachers and students. Skitch belongs to Evernote's collection of handy tools and is a free photo annotation service available on iOS and Android mobile devices as well as Mac and PC computer platforms. On mobile devices, you can capture and annotate photos, screenshots, maps and more with text, shapes, callout arrows, and labels. You can then easily share these photos through your Evernote account or Camera Roll.
As mentioned above, it is also available for your computer too. I often use it for student and teacher tutorials to call out certain icons, buttons, windows and more when explaining how different programs work (see below). While you don't need an Evernote account to use Skitch, having one means you can save your Skitches and archive them using Evernote's awesome organisational tools. Besides, Evernote is free and well worth having in your teacher's toolkit.