Looks like ImageShack did change the way the link choices are presented.
To display a full size image, copy the code line under:
"Embed this Image"
and copy the code line for "Forum" (NOT Alt Forum)
To display a thumbnail linked to a full size image:
"Embed thumbnails of this image"
and copy the code line for "Forum"
Remember you can resize images during upload to ImageShack by checking the box to resize and choosing 640x480 or smaller.
Your "Testing Area" forum post had thumbnail code:http://www.oldgas.com/forum/ubbthreads.php?ubb=showflat&Number=166449
Hope that helps.