I think there are several methods, but here's one:
In the red bar near the top of the screen, to the far left you'll see "User CP" (User C
anel). Click that.
You'll notice a bar near the left of the page. At the bottom, you'll see "Attachments" (Under "Miscellaneous"). Click that.
Then you'll have a box that says, "Attachments posted by (username)". There are check boxes to the right of each attachment you've posted, then at the bottom of the box there's a button that says "Delete Selected". Click that and hope you chose the right ones to delete.
To avoid that issue in the future, use a site like Photobucket or the like to link pictures from. Then your only limit is the size of your account on said photo hosting site. Or if you use a site like facebook, you can link your pictures from there. There may be a security issue with that, however...