WinXP SP3 here and same thing with that png. Searched the cover with the api commands and saved it from Opera = correctly saved. So it's probably .NET in WinXP.
While I was searching the same cover from the last.fm site (couldn't find it first (from the webpage).. then used the api method), I stumbled onto something good: last.fm has (for some albums, tested only couple) 600x600px album covers too! They're just "hidden".
Found it when checking the source of the Zauberberg album webpage (http://www.last.fm/music/Gas/Zauberberg). The revealing bit:
"original":"http:\/\/userserve-ak.last.fm\/serve\/_\/41520903\/Zauberberg.png"
->
http://userserve-ak.last.fm/serve/_/41520903/Zauberberg.png
And when later checking/testing out the api (image) results, discovered 2 alternative links:
http://userserve-ak.last.fm/serve/_/41520903/41520903.png
http://userserve-ak.last.fm/serve/_/41520903.png
Now, the api returns the following info (for this album):
<image size="small">
http://userserve-ak.last.fm/serve/34s/41520903.png
</image>
<image size="medium">
http://userserve-ak.last.fm/serve/64s/41520903.png
</image>
<image size="large">
http://userserve-ak.last.fm/serve/174s/41520903.png
</image>
<image size="extralarge">
http://userserve-ak.last.fm/serve/300x300/41520903.png
</image>
I guess it would be "relatively easy" for a BOO expert to add a check if the "original"/biggest cover exist at all and choose that cover instead.
I seem to enjoy "detective work".. too much.
P.S. Compared "hidden" Zauberberg 600px last.fm cover to the 600px (jpg) Album Art Exchange cover and they seem to be identical, except gamma/brightness. Interesting.