One of the lesser-known features of Google Drive is the collaborative aspect of Drawings. Drawings in Google Drive 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.