If they were ready to make a decision within thirty days, they would not be asking for photos which are proof of relationship or birth certificates, both of which are crucially important to the application. It can take 30 days for a visa officer to even see the files, never mind decide if they meet the requirements.
If a person receives this letter, it should not be interpreted as "my decision will be made in 30 days." It should be interpreted as it is written - you must provide proof of relationship. Photos are part of that. If you have not provided sufficient photos to prove the relationship, CIC sends you this letter demanding more. They give you thirty days to provide the photos, or else they will make a decision on the information they currently have, whhcb they have decided is not enough because they've asked for more.
Please stop suggesting this letter is a sign that the application is almost finished. It isn't necessarily, in fact, it means there is not enough information in the form of photos to make a positive decision.