But to be just a bit more specific, you do NOT upload your pics to the forum. What you do is upload them to a photo hosting site (like Photobucket...there are many others as well), then link to the pic from your message using what are called IMG tags.
To automatically generate an IMG tag, click on the "Insert image" button (the yellow square with a mountain in it) and enter the URL of the image you want to link to in the window that pops up. In your message, you'll see the URL with the tags attached. (All the button does is keep you from having to type the tag yourself.)
Photobucket makes this even easier. Once you upload the pic to photobucket and view it, you'll see a bunch of URLs below it that you can copy. One of them is the URL of the photo with IMG tags already attached. Just copy it and paste it into your message.