Snaggy a try.
Snaggy is a web-based tool for drawing on, annotating, and sharing screen captures. To draw or write on your screen capture just paste your screen capture image into Snaggy. Snaggy offers tools for highlighting a section of your screen capture, typing on it, and drawing free-hand on your image. You can also use Snaggy to crop your image. When you're ready to share your screen capture, Snaggy assigns is a custom url that you can Tweet, email, or post anywhere you like. Snaggy lets you save your edited screen captures to your computer too.
Applications for Education
Snaggy, like other screen capture tools I've shared, could be a great tool to use when you need to add directions to a screen capture. Whenever I run a workshop in which I'll be introducing people to new tools, I make sure that I have annotated screen captures to share. Those screen captures make it easy for people to go back and get instructions if they miss a step during a demonstration or if they need a reference after the workshop.