The following is from the Merlin Annual Pass FAQ section:
The Annual Digipass allows guests to collect all of their digital photos from selected ride, green screen and walkabout locations at THORPE PARK Resort, Chessington World of Adventures Resort, Alton Towers Resort and LEGOLAND Windsor Resort. In addition to your digital photos you are entitled to £5 off the price of a standard print photo when you show your Annual DigiPass at these four Theme Parks. Discount cannot be used in conjunction with any other offer.
The annual digipass is £50 and is only available to Merlin Annual Pass holders.
The way it works is you tell the staff member you want to buy one and they issue you with a temporary pass, a wristband and a form that you need to fill in. Take those three items with you to the specified point (at Alton Towers it's the walkabout photo collection booth on Towers Street) where they will issue you with a proper pass.
Once you have it all you do is tell the Picsolve staff member which photo you would like and then scan your pass (only photos in which you appear can be associated with your pass). If you have the Picsolve app on your phone then you scan your pass using the built-in function and all your pictures will be automatically there. Photos can take up to 24 hours to appear but most of mine were visible in minutes.
If you visit the parks often enough then it's a worthwhile investment.