To post a photo in the forum it must "live" somewhere on the net, first. Some people use flickr or photobucket. If you have your own website and it is posted there, that will work also.
Once the photo is uploaded to one of the sites mentioned above look at the bottom of the box where you write in your post and click on the bar that says "switch to full reply screen".
When you are in that screen there are buttons across the top of that box and the fourth one from the left is to enter an image. Click on it and follow the prompts.
Good luck, I'm looking forward to seeing your photos.