If you have a Silver/Gold account, you'll see an option to "Draw Feedback" on your students' Show Your Work responses. Click on this button to launch the full-screen canvas.
You can use any of the Show Your Work tools (draw, circle, create a square, draw a line, type, upload an image, use the Math Keyboard, select, erase, and back) to annotate your students' responses. When you click the checkmark, your annotations will be saved as an image that you can send to your student instantly as a feedback message!
Your students will be able to zoom in on the image when they receive it to see your annotations in great detail.