Once your photos are on a photo hosting site (such as Imgur.com;
Photobucket; Postimage; 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, which should end in .jpg) into the IMG box in the reply window of the thread you're posting to.
The site's album is also a good choice.