First of all, you have to store your photo somewhere on the internet.
You'll need to set up an an account for any free photo storage site - that will take you about ten minutes. Once you have an account you can access your photos in seconds if you remember what you called them or if you add tags.
Then download your photos to Flickr (That's what I use) Photobucket, Picasa or whatever.
Once you've done that, go to the photo you want, right click to get the photo address then above the posting box in the forum you'll see "Img", press that button and press ctrl/v put the address of the photo into the box and hey presto it will turn into a photo when you post.