I assume you mean on sites that are NOT TGP galleries...
TGP owners hate it..
but for freesites, blogs, etc, I can't understand why the program owners would get upset.. after all, they're already providing the FHG, why can't you use a thumb from it as a preview???
I've asked a few sponsors, and they don't have a problem with it IF their security can allow it
They're never willing to change the .htaccess though