When you are posting to a thread to add pics, scroll down below the posting box and you will see a button that says 'host it!'. Click on that and it will take you to a page by imageshack; click on 'browse' and find the pic you want to post and add it, then click 'host it' on this new page. You will wait a few seconds to a few minutes (depends) and a new page will come up with a thumbnail of your pic and several options.
I have found that the second option (thumbnails for forums (1) ) works best here. Just right click on the link to highlight and choose 'copy'. Then all you do is right click/paste in your posting box where you want to pic to show and you are done.
Remember to preview your post to make sure it works fine before you submit it. When you read your post you will see a thumbnail pic. Forum members can click on this thumbnail image if they wish to see a larger pic.