Well, it's two basic steps.
1- Enter the site below and upload your image. It will create a link. Copy that.
2- Start a reply. Write "[img]", without the quotation marks. Paste the link you just copied, no spaces between the link and the [img]. Then, with no spaces either, write "[/img]" (no quotation marks, again). Click Preview Post. If your image shows big, than you're good to go, and can press Submit. If not, check again what you wrote.
An example of what you would write: [img]blablablabla[/img]
Being "blablabla" the link you copied in the imageshack site
I hope that helps.