Re: picture signatures

While timestamps do have some use for pictures, they're more limited in
what they'll do for you.  Paul Baclace suggested that if you have the
oldest secure timestamp for a given picture, then you can prove it's yours.
Some problems include:
- NOT having the timestamp doesn't prove it's NOT yours, whereas a signature
	is real good evidence.
- If somebody changes one or more bits of the picture, your timestamp is
	no longer valid; the same is of course true with signatures.
	But with signatures, you can demonstrate *who* you got the picture from,
	whereas timestamps don't do that very well (though I suppose you could
	accept signed timestamps as well as acceptingsigned pictures.)
- Timestamps are more anonymous, but you can achieve the same effect with
	signatures by creating a random public-private key pair to signing
	each picture, and then demonstrate knowledge of the private key
	if you need to prove ownership.

On the other hand, secure timestamps *do* give you timestamping, which
signatures by themselves don't, so it's certainly a valuable addition.

		Bill