It seemed to me also that Categories would work well for a photo album. The model is simple: An Album belongs to zero or more categories. Photos don't belong to categories, they only have keywords and attributes which describe them, not categorize. Search would be used to find photos related to keywords.
That said, one design idea that crossed my mind is to use Categories to store the keywords, by creating a new category for each new unique keyword. I havn't decided which is the better design.
