Under this box there is a whole lot of information, then there is a blue bar saying Add an Attachment.
In that section, click BROWE to look for the file on your computer that you are wanting to attach.
If you want to add a comment, add one in the file comment box, but you don't have to.
Once you have done that, click add attachment. That'll refresh your screen, but won't delete your message, and voila - one picture added
Hope this helps