I'm sure there's some reason -- but whatever it is, you might want to point it out more clearly on the front page, because otherwise those 10 seconds are probably all you're going to get from most people.
"No account needed!
It couldn't be easier! You can share photos with friends and family right now!"
You can tell it's important because it's repeated again directly below, with a minor change in wording:
" Share your photos NOW - No account needed! "
That's actually huge, I'm guessing it's even the main feature that sold YC on accepting them (to me, "photo sharing made stupid simple" = "no account needed") especially considering that trend in recent YC startups like posterous.
Actually, posterous is a great example, go look at their home page, the super prominent crossed out "make an account" line at the top of the page gets across the point really nicely.
I always try to build my apps so that you can use most of the functionality without an account. I think that should be the standard way to do it and not other way round.
Add a tagline to the divvyshot logo in the top left.
The only advantage over flickr I can see is that it's free - but even that is apparently only for alpha.