which version are you using, I even saved one with ctrl +s and no problems at all.
I'm using Qimage 2008.107 here. I don't think it's a Qimage problem as such, it's just brought the issue to light through it's hideously fussy standards for displaying formats and error labeling anything it doesn't like.
It looks like the default file options are varying somehow in CS3, sometimes Ctl+S works fine, sometimes it doesn't. In the same way, when the file options dialogue opens when saving a file, the default values aren't always the same.
I'm not sure if there's a way of definitively setting the defaults somehow.
CS3 has been a pretty disappointing version of Photoshop overall. The printing options work more poorly on Windows than in CS2 and before, plus the Bridge 2.1 needs some serious work to get it working quickly enough to be useful too.