You need to use a photo hosting site...photobucket, imageshack...there are quite a few.
Follow the instructions on the site...basically you upload the picture to it, and it will provide you will a code that you copy and paste into your forum post.
It will inevitably look a bit like this....
[img ]www.imageshack.com/blah blah your image . jpg[/ img]
Its the URL of the image, with coding at each end...
<a class="postlink" href="http://thetotalfootballer.com/wp-content/uploads/2010/10/carlos-tevez-manchester-city.jpg" onclick="window.open(this.href);return false;">http://thetotalfootballer.com/wp-conten ... r-city.jpg</a>
becomes
[ img]http://thetotalfootballer.com/wp-content/uploads/2010/10/carlos-tevez-manchester-city.jpg [ /img]
and therefore...