I don't know for sure if old pics will be deleted when you try to add new ones. You will most likely get an error message that says your attachment limit is full.
To delete old pictures, and also view how much space your current attachments are using, go to your User CP (look in the blue navigational bar at the top of the screen). In the left column, find the Miscellaneous block (all the way down at the bottom of the column) and click on Attachments.
You'll see a list of all your attachments, the thread they belong to, and the date you added them. To remove an attachment, click on the check box to the right, scroll down to the bottom of the page, and click on Delete Selected.
The thread links are clickable, or you can also click on Show Thumbnails (at the bottom of the page ) to help you decide which pictures to delete.
