Aha! Oh my, this used to be one of my specialties :) First of all, if you are looking for better ways of handling stylesheets in NN4, let's dig up an old thread, shall we? [webmasterworld.com...]
Now, it shall be added that you are in for an especially difficult challenge. I haven't dealt with NN4 for so long, and quite frankly do not have a way of testing in NN4 any more (and no, I will not install it, thank you ;)). But, what I do remember is that heights and widths are, at best, shady in NN4. I can't remember the specifics about whether image dimensions can be set the way you describe, but it is very likely that the answer is "no".