An easy way of letting your students upload files to their test is by using a QR code. Students will have to scan a QR code with their phone, and upload the picture directly into the test.
How to enable this option
In the test settings, under "Students can upload files during the test" choose "Students are allowed to use an external device to upload a file.
See how the students see it here.
Watch the video example on how to enable the QR upload (per question) and see how it looks like in the test.