Forum Moderators: open
A 1x1 image, whether invisible or not, is a valid design tool...a 1x1 invisible link can never be utilized by a real person and therefore is spam.
Peter
Since you are using the images for mouseovers (for which you need JavaScript anyway), the best approach is to preload with JavaScript as well (as soapystar suggested). If the user does not support JS (and therefore won't see the mouseovers!), you'll have wasted no bandwidth by preloading, either.
If you *do* use 1x1 GIFs for formatting purposes, using alt="" is the right thing to do (make sure they are not linked, though, or you may get into trouble with the SE).
However, unless you have to support *really* old browsers, using CSS for layout may be a better (and faster) solution.
HTH!