Cool. I looked at them all and the only other bmp that had a failed attempt at transparency was the hold icon for the gigabeat, which I promptly fixed and then replaced the zip on the wiki (without noting it on the page).
I also re-replaced the 320x240 (5g) zip because I realized I originally uploaded it with a Thumbs.db file in the bmp directory. Whoops.
Also, if anyone with higher access on the wiki is reading this, can you delete the "Ultima_320x240_v1.01" attachment on the 5g wps page? That was how I uploaded it before I decided it would be a better idea to just leave the filenames the same.