The skin for user 5f9fa6d2-dc48-420e-b7c8-e8c82c817282 is currently http://textures.minecraft.net/texture/49c7c25e26655aa7244125fb43b2ad603e5ba123677dc9b575bf7301ff2ff.

The server responds with

HTTP/1.1 200 OK
Content-Type: image/png
Content-Length: 1832
Connection: keep-alive

����JFIF��XExifMM�i&��@� ��C
...

Obviously that is a JPEG (JFIF), not image/png.

You could correct the Content-Type header, but IMO that's the wrong approach as pretty much everything probably expects a PNG here.

I attached a file with more of these, some are GIF, some BMP, most are JPEG.
I'm sure there are more, however. To find them, you can query your database for blobs that don't start with the 8 magic PNG bytes 89 50 4e 47 0d 0a 1a 0a.

If that is a problem for you, I'm willing to help out a little by reporting more skins that are not PNG (we make a lot of requests) so you can manually fix them.

You could also solve this issue alongside WEB-27. You'd just make sure they're converted to PNG. Alternatively, you could check the magic bytes when you deliver skins and simply delete them when they're not PNG.
I'll leave the solution up to you guys

PS: My favorite pictures are http://textures.minecraft.net/texture/3e453c5a747c219cab738bb573556317d1a405cf1886c16b995fd86f2e05399 (apparently someone uploaded their desktop background) and http://textures.minecraft.net/texture/a5587f9eb25cc312119269dd239ff94a49c36ae47d635c33ca184e5b7d842 (some picture taken off eBay, it even has a comment: 'Processed By eBay with ImageMagick, R1.1.1.M2')

(Somewhat) related: WEB-27, WEB-119