In deploying SIG on Joomla 1.5.8, I noticed that a few pages would show thumbnails, but some wouldn't.  In both cases, clicking where the thumbs would be produced the image.

I found out, through experimentation, that if the filename contained an apostrophe, the thumbnail wouldn't show up.  As soon as I removed the apostrophe from the filename, the thumbnails began to work.

I haven't had a chance to dive into the code, but plugin users and developers may want to take this bug into consideration.

Hopefully this may help some of you out!

-Joe