As a regular member you are allowed 10 megs of photo space. I see that you have almost 70 attachments uploaded and some of them are quite large. In order to add more pics you will need to do one of the following:
Go into your UserCp, click on attachments and start deleting some of your older pics thereby freeing up some additional space.
Start using a photo sharing site like photobucket. If using one of these sites remember that if you delete the pic from that site or if there is some other glitch that occurs there, the pictures will also disappear from the forum.
or, (and my favorite)
You can become a Supporting Member here on the forum which will not only allow you 10x the amount of space, but you will also get some really cool forum stickers, a larger avatar, you can change your username blue and you will have access to the Supporting Members Only forum.