Go to user cp (as pictured above)... then manage attachments (it's down the list on the left). This will show all pictures that you've attached to posts. You'll have the option to delete each one. Just know that any you delete will be removed from past posts.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry