below the text box, you obviusly can see
additional options, to the right of it, you have the
Imageshack toolbar, you simply
browse for the picture and click
Host it once the picture is uploaded-hosted, you copy-paste a link, from the new window that poped out. the best choice would be
thumbnail for forums, either 1 or 2, if the first one doesn't work, you try the second one

i think your problem was that you closed the pop-up window.
i may have complicated a bit, but i hope it helps.