If you look at 'Additional Options' under the Reply To Thread box, you'll see an 'Attach Files' options. But there are strict size limitations. I haven't tried this myself yet, on this forum, so I'm not quite sure how it works.
Anything bigger than that, or if you want more flexibility as to how you place your pictures, you can use one of the various free hosting services and then just post the URL between IMG tags, somewhere in the text. I use Imageshack which is quite quick and easy.