You can either post a comment against the entire image asset, or post against a specific area in the image asset.
In the Insert a comment box, enter your comment, and click the + icon.
Your comment is then posted below.
In the Manage Image view, click the + icon, located on the toolbar beneath the asset.
Hold the left mouse button down, and drag the cursor across the image until the square has covered the desired area.
Enter a comment in the comment box, and click the + icon.
The comment is then posted against the selected area. If you click on a specific comment in the list, the area that the comment was made against will be highlighted.