Ok; thanks. I've not noticed a problem using that method, but Impress does seem to have more than it's fair share of glitches.
I'd still like to see a sample, if possible. You can chop opne of yours down to just a few slides and see if it still shows the same problem; if it does, either attach it here, or use one of the free file-sharing sites (mediafire, rapidshare, etc.). You can use the "Upload Attachment" link (below the message entry area after you click "POST REPLY"). [Forum] How to attach a document here
Hmm, I think I see what you mean: even though I answer "no" to the "Change background for all slides", the background image changes for all slides.
Is that the problem you're seeing?