Sounds like you're doing a revealed audit on the purple portal. If your phone takes clear pictures with sufficient resolution, there should be no problem. If not, you won't get paid.
I have always used a camera.
If the job description says that a time/date stamp is necessary, make sure that you use the app to add the stamp. Others have done so effectively - you could search this forum for several discussions re. adding time/date stamp ethics and processes.