Yes, there is plenty of room even in small images to post reviews and comments. You can zoom upto 400%, making commenting easy.
From the dashboard, double click on the uploaded image to launch the proofing window. To add a comment, click on the rectangle box or on the comment box at the top left. Add the annotation or draw the box anywhere in the image where you want the changes to be made and type your comment.
Clicking on the comment box in the image, highlights the comment in the right bar that includes the name of the reviewer and time when the comment was made.