Hi Laura you need to go to Members tools them click on Your storage information .
Now choose any group that you have pics in ( select A Group ) then click on My Pictures or My Attachments and from these you can remove any pics you've added in groups .
A good idea is to just remove by dates deleting oldest Pics / Attachments first as once attachments are removed they disappear from your messages .
Hope this helps .
By the way did you notice I set you up a message box ?