I often watch lectures on my phone. As far as I can tell, there is no way to take a QR code from a phone and upload them on venue.
It would be nice if I could screenshot the QR code, and upload a screenshot of the QR code to venue that would make it much easier for mobile users to use the QR code feature.
I understand there is a risk that you could share the image with friends, but you can already do that. And what you could do is get a hash for each image uploaded and make sure that no hashes match between 2 users.