Make sure temp file prefix is at least 3 characters.

Started out figuring out why PNG did not worked, turned out it was an arbitrary requirement of the Java language...

http://docs.oracle.com/javase/6/docs/api/java/io/File.html#createTempFile(java.lang.String, java.lang.String, java.io.File)

While I was in there I also did some cleaniing up and turned on some switches by default.
Merge pull request #2442 from guardian/resize-png

Make sure temp file prefix is at least 3 characters.