As long as they are in your system then that is easy, but if already loaded on here?
Are you asking about photo's that you have already uploaded here and that are in a topic ? If so just open the page that you want to delete pictures in. Click on the "modify" tab on the upper right side, click on it. That will open the page in question. Then expand at the bottom where the pictures are uploaded, (attachments and other options). With that open you will see the info only on the pictures, go to the left and you will see a small box to that picture. Just un-check those that you want deleted and then click to close.
Hope this helps.