I've juryrigged all the logos around the joint to be changed simply by changing File:Wiki-wordmark.png. This means that in some cases the image is very slightly fuzzy, because it's being stretched. It's not pixellated anywhere — just not quite crisp. But it's more than good enough to check its performance.
Please check it out in Monobook, on the Main Page, at a typical user welcome message, on a dark background, on a light background and report your findings here.