First, how do you make money? Honestly, I would charge people for this and have premium features like hi-res images.

What is the killer feature that make me want to use this instead the many "good enough" alternatives that I already know well?

I wouldn't build this in code yet. Mock it up. Show it to your friends. Is it better than what they currently do?

Would they pay for it? If they say, "Sure, I'd pay", ask for the first 6 months of the subscription right then, a check will do. You'll return it if you don't build it, but this is the real test.