I think I found a solution, using some ideas I found here:http://highslide.com/forum/viewtopic.php?t=1483
I used the addition provided by 'java88' but used "hs.getPageSize()" instead to determine the page dimensions and changed the loading label position from "absolute" to "fixed". So far, it seems to work just fine in all current browsers, except IE6 (of course). But we dropped support for IE6 in order to utilize some more modern features on our website. I also changed the loading label itself to have a bigger image (CSS; .highslide-loading).
Again, thanks for the awesome and quick support the forum is providing!