I fixed the cow
I finally went back to the source and got another copy of my cow image, recolored it purple (the original is in shades of brown; if you have MS Office you've probably seen it), and resized it correctly, so it now looks sharp, not distorted, mushy and pixellated. I even crawled forward into the nineties and saved it with a transparent background. While I was at it, I created two variant cows with slightly different color schemes, so if I should go back to a dark background for the sidebar (I'm considering it), one or the other will still be visible against it.
Yeah, I know, big whoop. Some people spend less time on a site redesign than I spent on that cow.
I'm just waiting for someone to say, "Don't have a cow, man," so I can reply, "Too late, already got one."