First, you have to upload the images you want to post to a web-server. There are still a couple of free servers in existance. If you use AOL, you have automatically received some space for uploading with your signing up to their services.
Once the images are uploaded, they have received a unique address, i.e. "http://members.aol.com/yourname/picturename.jpg". On the forum you want to post the image to, open a new thread and press the "IMG" button (It's right above the text-window") and enter the address of your picture into the pop-up field that apears. Voila!