I would imagine a high quality picture would do fine. Seems they're gonna scan the QR code. If you're supplying the invoice from OCuk with it, I'm sure they can tell its your card and not a pic from google images.
Edit:
Just tried to claim my "AMD Rewards: Battlefield 1 Deluxe upgrade pack" only to be notified that this promotion expired on November 30th.
However, the webpage where the card is for sale has no mention of expiry.
https://www.overclockers.co.uk/msi-...cs-card-20-cashback-gx-32j-ms.html#free_items
The card is being offered with a game code which don't exist anymore. I'm sure this was just an oversight by OCuk, but am dissapointed that I paid for a card claiming to come with a code, when in reality it doesn't.