I am not an expert in copyright law by any means, but this really seems like it is blatantly breaking copyright and IMHO is not ethical.
You can't just take somebody's possession, tell them if they want it back they can ask for it back or else you will do whatever you want with it, and think it is okay. That is what it seems you would be doing with this website.
If a person takes an image with their camera, that image is the property of that person, and they own the copyright to it. You cannot use it without their express permission, I think.
Watermarks and logos are placed on the images to protect the owners, and should never be removed by someone who just wants to use the image for their purposes.