There are a couple ways to do it. One is to click the "Manage Attachments" button down below the text box when replying in a thread, then uploading your pics straight from your computer. There are some size limitations to the pics, though, so check to make sure that isn't your problem.
Another option is to load them up to a hosting sight like Photobucket. Once you've done that, just copy the "IMG Code" string, and past it into the text entry box here with whatever text you are typing in.