My libravatar has a transparent background, and libravatar.org correctly
preserves it on resize, but bugs.debian.org replaces it with black.

http://cdn.libravatar.org/avatar/15c74c0974c6cc25c194a2737e2d6747
http://bugs.debian.org/cgi-bin/libravatar.cgi?email=andersk@mit.edu

Anders

--2013-11-16 22:07:31--  http://cdn.libravatar.org/avatar/15c74c0974c6cc25c194a2737e2d6747
Resolving cdn.libravatar.org (cdn.libravatar.org)... 5.35.241.22, 166.78.0.128, 176.58.122.199, ...
Connecting to cdn.libravatar.org (cdn.libravatar.org)|5.35.241.22|:80... connected.
HTTP request sent, awaiting response...
  HTTP/1.1 200 OK
  Date: Sun, 17 Nov 2013 06:07:30 GMT
  Server: Apache/2.2.16 (Debian)
  Last-Modified: Sun, 17 Nov 2013 06:00:26 GMT
  Accept-Ranges: bytes
  Content-Length: 12569
  Cache-Control: max-age=604800
  Expires: Sun, 24 Nov 2013 06:07:30 GMT
  Access-Control-Allow-Origin: *
  Keep-Alive: timeout=15, max=100
  Connection: Keep-Alive
  Content-Type: image/jpeg
Length: 12569 (12K) [image/jpeg]
% identify 15c74c0974c6cc25c194a2737e2d6747
15c74c0974c6cc25c194a2737e2d6747 PNG 80x80 80x80+0+0 8-bit DirectClass 12.6KB 0.000u 0:00.000

So basically, libravatar.org doesn't actually give a useful content
type; we assume that it's content type is correct, and turn it from a
PNG into a JPEG which ditches the transparency.

This is a bug in libravatar.org's CDN, though perhaps the BTS can do
better and ignore the content-type given and actually calculate it
itself.

I just filed a bug on the upstream tracker for this:

https://bugs.launchpad.net/libravatar/+bug/1252037

If you have ideas on how to fix it from that side, please chime in.

Francois