If you attached the images, edit the post and open the download manager. You can remove the specific images there. Also the "Thread Tools" (up and right on every thread you start) make you able to delete or close threads (at least sometimes). You may have to contact a mod for that.
If you go to your user Control Panel (right above the Post Reply button at the top of the thread), at the very bottom of that there's a menu item called "Attachments" where you can delete anything you uploaded here.
To delete the thread you only have to delete the first post in it. If you are having trouble, send a PM to one of the mods for that forum, tell them what thread you want deleted and they'll take care of it. (If they don't respond in a day or two try a different mod, there's a few less active ones.)