To leave a comment on a individual picture, click on the picture to enlarge it. When the large version of the picture comes up, under it will be a link that says [add comment]. Click on that to leave a comment about that picture. 
Once a picture has a comment, a red * will appear by the picture, which means it now has a comment with it. Just click on the picture to enlarge it and all comments for it will come up with the picture.

And thanks!