The URL editing Javascript automatically strips the string "http://" from within URLs. This makes it difficult to enter certain URLs, such as those occurring on archive.org:

http://classic-web.archive.org/web/20110704084740/http://3.bp.blogspot.com/_nSuPj1LEQDA/Rdi0YWkHJSI/AAAAAAAAAGk/YZq2ncz6P1w/s1600-h/front+cover.jpg

I think the missing http:// is what makes http://musicbrainz.org/release/9a5be08f-bb76-4a8c-824b-7e1d225e0fbe not show the coverart.
For some reason, it doesn't always prevent the cover art from loading, e.g. http://musicbrainz.org/release/5ef08648-db92-4c12-b0a2-abac6081778c/relationships.

Note that it's possible to get around the limitation by disabling JS (e.g., with noscript on firefox). Assuming that embedded http:// is valid in URLs, I think it should be allowed.