Basically, the code for posting images is the same as what you were already doing, but it has to be an image you are hosting yourself on your own image hosting account (on Photobucket, Imageshack, or whichever you like). It can't be from anyone else's site or image hosting account, because then you are stealing their bandwidth.
When you see a picture you want to share, right click on it to save it to your computer, go to your image host, upload it to your account, and then post it to a forum.
Pretty much any forum you go to will have rules against hotlinks, so this is a good habit to get into.
Thank you for giving me the instructions so patiently. I'll insert the pictures in the top post, tell me if i do that wrong please!