MD5 is perfectly fine for this, the odds of accidentally stumbling over a collision in real world data are insanely small. They're not going to prosecute you for it, it's obviously used for flagging up images.
That being said you could find your image deleted from facebook or something - but it's not as if you won't be able to say "well this is wrong".
When I say md5 is fine I mean on a technical level not using file hashes is generally a stupid idea for this purpose for obvious reasons, but sha-512 isn't exactly going to improve the process - there's less theoretical collisions but..
Remember you're talking about 128 bit key space so..
Biting the hand that feeds IT
