Right now we set a cookie and then check the cookie to see whether a user has JS enabled. This breaks when the user turns off JS after the cookie has been set (you have to clear the cookies for it to realise that JS is no longer on) and means that after doing that, all cover art images fail to load until you turn JS back on or clear your cookies.
It feels like we're reinventing the wheel when HTML already has a <noscript> tag for this sort of thing. Unless I'm completely missing something, we could do something like <script>do complicated shit to determine which image to use</script><noscript>insert 250 thumbnail and hope it's been generated</noscript>
I realise this isn't a high priority thing to fix, but I'm reporting it anyway because the way we're doing JS detection is buggy. : P