Hey, you use this: [ I M G ]http://url.to.your.image.com/image[/ I M G]". Remove all spaces in those I M G things. Just make sure wherever you are posting your picture from that you resize it to forum guidelines.
However, this requires that you have a website where you can put your pictures, and then link to. Some people have their own private servers, others use payed for services like .mac, and others use free services. I am only familiar with .mac, and setting up a private server, other members should be able to point out a free service for hosting pictures.