That you're asking about keeping it unguessable makes me think that it feels like too few. Its actually only 5 bytes of random (the first character is always zero). 2^40 is a pretty large number and I don't expect to get high coverage over that space any time soon. That being said, perception is important, so I should probably up it.

As for hot-linking; I have no business plan and just wanted to make something useful. I should probably figure that stuff out.