One of the lesser-utilized features of Google Drawings is the ability to comment on images. Drawings allows you to collaboratively create drawings from scratch and or alter images that you upload to Drawings. By uploading an image you can draw on it and write on it to add labels. Google Drive Drawings supports commenting just like Google Documents. The sharing options in Drawings are the same as those of Documents too.

By using the collaborative drawing tools you could start online image-based conversations with your students. You could also use these tools to have your student collaboratively label diagrams. Directions for these processes are included in the screenshots in the slides below.

Watch this video for five more neat things you can do with Google Drawings.