I'm a big fan of delivering utility with less barriers. It probably is worse for your "active users" graph, but I always feel warmer and fuzzier about apps that

I'm a big fan of delivering utility with less barriers. It probably is worse for your "active users" graph, but I always feel warmer and fuzzier about apps that I can engage with at my own pace. So, in this case, that's an argument for attaching a reasonable-sized copy of the image to the email, or maybe a bit of embedded HTML with an obfuscated link or something that redirects if you want better tracking of what's going on / privacy knobs. Sure, some people will never register for an account because they'll get all their utility out of the emails. But on the other hand, all sorts of non-tech-focused family members will get to see the artwork, which is what your users want in the first place. Glympse stands out in my recent memory as an app that did a great job of this.