Hosted Free Sites and Galleries
 

I am wondering if it is ok to download an image from a hosted free site or gallery to make a thumb for the free site or gallery on Your category page?

easy ...ask the sponsor for permission.

But there is no general rule that it is ok or it is a nono?

Technically it's a no-no. Your domain would not be covered under the content purchaser's license. You might instead direct your submitters to include a url where you might find a thumb for you to hotlink.

Look at all the thumb tgp's and thumb mgp's... that's excactly what they are doing. Taking AND in most cases editing/cropping a thumb from the hosted galleries.
It's become standard even though emmanuelle is indeed right that your domain is not covered by the license.

Yet another case of "if everybody does it then it must be okay"

yup, i agree... just ask permission dude...