Here are 3 things in Google Docs you may not know.
1. Images in a Box
2. Suggest Edits
Suggest edits allows you to directly edit a students document with suggestions. The student would need to approve the edits, or they can say no.
3. Insert Drawing
You can use inserting drawings to allow you to annotate on an image. Students are able to drag images to the drawing canvas in the Google Doc. The image can be cropped if desired. Students can insert arrows and shapes onto the drawing, as well as text boxes.
To edit a drawing inserted into a Google Doc, simply double click on the drawing.