I really don't think this is a fair competition to be honest, specially that you've announced it this early.
Why? because someone out there is writing AI code that does character recognition in images and will sweep the whole thing within couple minutes.
Even without it, since you've posted the image without the modifications, it's possible to create a delta-image (difference between 2 images) using the current image and the one after adding characters which will reduce the amount of lookup to do.

Don't get me wrong, the idea is awesome in theory, but in practice it's hard to implement in a fair way.