You basically need to host the picture that you have on your computer somewhere on the web. There are plenty of websites that offer this service for free, such as
www.photobucket.com,
www.photojerk.com and
www.imageshack.com just to name a few. You also have the option to host your photos on this site for free! Each member here has access to a free web gallery. You can get to the gallery
here. Basically, all you have to do is click the "upload photos" link in the upper right corner of the screen. Once there, click on the "browse" button to locate the file on your computer. Then click the "upload/submit" button at the bottom of the page and viola! Your photos have now been uploaded. Your browser will show you the uploaded photo with some code underneath it. You need to paste that code into the message board here to post the photo, just make sure it is within the size allowances (no more than 640 pixels along the longest edge). The code will look something like this:
[img]http://www.websitename.com/imagename.jpg[/img]
There's also a tutorial on this website on how to post photos if you need more in-depth instructions:
Photo Posting/FAQ Tutorial.
Hope that helps!