Sat Jan 05, 2013 8:49 am
MD5 and SHA-1 are different hash algorithms and will always produce different results. MD5 is a 128 bit hash which is 32 hexadecimal digits, and SHA-1 is 160 bits or 40 digits.
f42b8a048d65ceced6ccb58ebdc8f533 is the md5sum for 2012-12-16-wheezy-raspbian.img, so your download is correct.
But if you are very concerned about security you should not take my word for that. The cryptographic hash is intended to verify that nobody has tampered with the image. So you should check it against the algorithm and value published by the Foundation only. Also note that the published hash is for the .zip file, not the .img.