For most forums, you only post a 'link' of the picture from the site where your picture is hosted.
For example, say I want to share an image of my hot tub. I go ahead and upload the picture I want from my camera or hard drive to Photobucket (there are other options too), then I copy the link of that picture (It shows the link by the picture) that is on Photobucket and paste it into my message here. I can do the same with video which may have been uploaded to Youtube too. So in reality, when you see a picture in a posted message, it's actually not hosted on this site, but is linked from another site (cleaver huh?). This process takes a couple extra steps but that's how most forums work because image hosting sites are geared up with lots of server space to handle photo storage, whereas these forums generally aren't. Hope that helps.