Hi,
This is not trivial but I'm new to FPGAs.
In order for the computer to recognize the DE0 nano as the x-ray sensor you are planning to plug into the DE0, you will need to configure the usb port in the DE0 with correct usb parameters of the x-ray sensor.
Then the DE0 will need to be able to capture the data from the sensor and store it in non-volatile memory. So that you don't loose the data at power down. Or RAM if you don't mind capturing another image every time you power down the DE0.
Then the DE0 needs to be able to retrieve the image (in memory at this point) and format it correctly to send it to the computer (via usb) when you push a button.
This could be a two step process. First plug the x-ray sensor into the DE0 and capture data. Then remove x-ray sensor and connect to PC for emulation. Either way ...well beyond my current capabilities. Good luck ... it looks interesting and fun.
As a side note. You may want to look at a new MAX10 FPGA just because you don't have to deal with the external configuration ic for the FPGA .
Take care.