when you make a thread, scroll down underneath where you write your text, go a bit futher down and there is a section for "Attachments" and there is a button "Manage attachments" press it and a new window pops up, in the new window in the top you select "add files" and browse your computer, then hit upload. If the pictures are not being used within 1 hour of upload, they automaticly get deleted, I believe.
When replying in thread, just press "Go advanced" button underneath quick reply box, to get to that same menu attachments site.