autocreate a thumbnail

Thanks to Kevin (mostly) and a variance of the Now Playing hack, I am able to upload images and display them with a caption just like CNN etc... in a news forum.

Now at upload attachment in the function postbit, I would like to be able to auto create a thumbnail with attached picture.

I understand how to alter the the thread table to add an extra field "thumb" in there and how to make a query to retrieve that thumb.

Can someone please tell me how to use the php image function (for GD), so I could include that statement in the posbit function. The final result will be displayed in the blank space here . I can do all that. It is only the creating the thumbnail part that is holding me. By what I gathered it is simply a matter of 2 to 5 lines of codes.

Thank you