In my limited testing what I observed is that it will cache some number of photos when it starts. I created a stream with 10 photos and each time it starts it downloads 9 of them randomly. I didn't do a lot of testing, so that number may have something to do with how much memory the device has. I was using an Express 3910 which doesn't have as much RAM as some of the more recent models. It will then cycle through those 9 photos indefinitely even if I kill the Internet connection.
Unless you have data caps you shouldn't really worry about it downloading the photos. Photos can't compare to the amount of data in a video stream.
Since you mentioned the microSD card, keep in mind that it's only used for channel storage. It doesn't have anything to do with caching photos or buffering videos.
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