First, start with uploading photos to a photo hosting site... I think photobucket works the best with message boards.
Once you have done that then you just right click on the image you want and select "properties"
Highlight and copy the URL address
The in your post hit the insert image button (the one that is yellow and looks like a mountain) and paste the url address in there.
It's as easy as that!! I know it seems complicated but it really isn't. Good luck