You won't be able to just upload a pic from your hard drive into the Stangnet forums. You will need some place to host it first. Like yousendit.com or some other free hosting site. Then you put a link in your thread (using the link tag [ IMG ]image[ /IMG ]) that points to the location where the photo is hosted.
You could also upload the photos to your "Garage" here in Stangnet and then link to them that way. I do that all the time.