Switch imageshack to classic view, it will display the code for forums both as thumbnails and for fullscreen (under the embeddable code section).
Otherwise, you can edit the code by hand. The code you got for thumbnails looks like this :
[URL=https://img207.imageshack.us/my.php?image=boom1bq3.jpg]
[IMG]htdp://img207.imageshack.us/img207/8563/boom1bq3.th.jpg[/IMG][/URL]
Remove the parts I highlighted in red.
EDITED according to sapi's suggestion below, thanks.
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