Here's a bit more detail:
Once your photos are on a photo hosting site (such as Imgur.com; Photobucket; Dropbox, etc.), or on virtually any site, select the full image, then Control-click (Mac) or Right-click (Windows) on the image itself, then choose copy image location. Now paste that URL (the full web address) into the box that pops up under IMG in the compose reply window of the thread in which you want the photo to show. Bingo; now you've posted a photo directly!