The video above explains how to work the desktop app version of Skitch. The screenshots below are from the mobile app version of Skitch.
There are many different tools to use including text, arrows, and the pen. Each of these tools can help you annotate your images by pointing out key ideas, details, and important words.
You can also change the color of the text, arrows, and shapes you add to your image. Different colors add a little pizzazz and help to sort and organize your annotations.
This is an image edited with Skitch by a student. Originally the image was just a black and white chart; she used Skitch to add color, highlighting, and text.
Once you understand the basics of Stitch, you can use it to annotate images as well as show your teacher you understand art, political cartoons, and text.