The email would preferably contain the image itself - an hourly shot of happiness. On my own site, I offer hourly/daily options for alerts, and a surprising percentage opt for hourly - I think you could do the same. It would get expensive using a third party mailing service like sendgrid - just sending mail from your server via Exim is pretty easy and costs nothing extra.

I have a feeling you could get a surprising number of subscribers, especially since the puppies would be super-forwardable...