if the images are not big(2-3 KB) and the collection of them is not supposed to be expanded then you can save them to db. But it would be harder to write them, read them. On the other hand the user won't be able to do anything with them.
Otherwise, if you want to let user to expand the image collection or the images are rather big you should store them to sd card.
Anyway, you should be aware of space lackness and treat those cases appropriately.
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