I had no clue that Amazon also has a Photos service. Probably you haven’t either, which is why they are offering free coupons to promote it.
You can view the offer on this page. I’m not eligible unfortunately, however the vast majority of my friends seem to be.
To get the £7 credit, simply download the Amazon Photos app and back up your photos, and you should receive the coupon within 7 days.
There is a minimum purchase value of £25 to use the credit. The promotion ends 28th February so don’t wait too long!