If you use [free] services like imageshack.us, they will provide links to you for use on forums after the upload is complete. I prefer this because you can post thumbnail links to images instead of posting the full size image on the thread pages, which sometimes stretch out horizontally (and we'd have to scroll right, including the rest of the text in the page). Not only that but it saves on thread-load-time since it doesn't have to load everyone's full size pic.

For example: