Publisher is not a great web editor. You end up with that issue so one workaround is to resize the image file directly with IRFANVIEW and not count on Publisher to do that work for you. This is not an offer to teach IRFANVIEW use.
I'm having trouble with some of my background images for a website I;m designing with 2007 publisher. When working on the document, the background looks like the way I want it to in that the images are the correct size and the background tiles the right way. Also true in web page preview. However, once site is uploaded and looked at online, the background images are huge! This is happening for a few pages I made and used background images from a particular set of clip art, whereas other backgrounds I used are working fine.
Additionally, the pages themselves don't take up the whole screen. When working on document, even though I have selected 984 pixels in width, the page shows up short of that. Therefore, things that appeared centered on working copy are off center in actual site.