Gmail could do that because "nobody" stored much e-mail. They could be on the case that most people would acquire more archived e-mail at a rate low enough that storage costs would drop quickly enough to cover a large percentage of the growth. In the end it also wasn't all that big deal when it came to customer acquisition: The major competitors all followed after they realised that it wouldn't drive their costs up all that much.

Pictures are different. They are already big. And everyone have lots of pictures. The main reason for signing up for cloud storage for pictures is that you have too many of them to store on your phone/tablet/other small devices, while for e-mail features, and network effects (all those people that knows your e-mail address) mattered far more than storage (consider that e.g. Yahoo charged to upgrade to 50MB or 100MB storage before Gmail, and almost nobody did - the free amount was sufficient for most users).

This is viable for Amazon because Prime customers are highly valuable and 1) we pay, 2) retaining us is worth lots of money - I've been up to over 200 orders a year from Amazon some years, 3) Amazon by now has years of experience driving down the cost of image storage, 4) it helps as a way of driving customers to their Fire platform (e.g. photos and videos in the Amazon cloud account show up on the Fire TV)