That is sad to pass off the work of others as your own. 

Well the prize you give is a big help but to stoop down that low is to disrespect the artist in you and themselves. 

Yes competition is fierce for your daily winners but to past off another person's photo is cheating. Maybe you should be fierce that if found to be guilty that they are blacklisted. May seem harsh but integrity is key.

I love looking at the pics that are sent your way because it does not only show exotic locales and subjects but also gives us a glimpse of the personality of a person.

As you said each one has a signature of how they take their photos.their framing, the way that they caption theirp photos shows us who they are.it is very personal. 

I support you whatever way you want to proceed.