Volunteers? And later, incentivizing/rewards?
I know that this is 100% you, Tom, from what I've seen, but poking around my own portfolio, and adding cards, I'm wondering where the error reports go? I hope they don't pile on you, but have you ever considered creating a volunteer team that can crawl Google images for the most common card for say, Topps - Base - 1995 - #320 when an error is sent in to show that it's obviously not part of that collection (wrong picture, etc.) - I am not a copyright expert, if that breaks some sort of copyright, or you could get around it by giving credit to the original and hosting it here (versus leeching their bandwidth).
Secondly, because you want clear, front and back pictures of cards your users actually own, what if later on if all of this starts becoming a stable platform (I don't know your bills!) - you could offer a 0.5, or 1 cent USD for a clear front, another for a clear back after being approved by said moderating/approval team (who would get 0.25 since it's easier to GIS a name/number/brand than it is for the user to take the picture, crop and format it, upload it, etc.).
And it doesn't have to be actual pennies, it can be a.. proprietary point system that you can offer rewards after x amount of points earned.
The reason I mention rewards is that there are people on PG.C that have thousands upon thousands of cards, and take a picture of each one should a picture not exist would be a large burden.
These are just my ideas, and I would love to hear from you if any of those interests you, automation and monetization are two of my strong suits for user-generated content apps and sites, and I've talked too much already, if I spouted off more it would be pages upon pages of thoughts. So I'll leave it at this for now
That said, I am also raising my hand to volunteer for the error response team (missing sets, incorrect images, etc.) should you want to try that out.