I have a photo website and i want to support tags as my original category bucketing is starting to fail (some pictures are family and vacations, or school and friends). Is
I suggest looking to see how established open-source software does it. For example, Gallery stores its meta-data in a database like you do, and is pretty rich.
I don't think you'll find a "standard" schema, though. The closest thing I can think of is the EXIF meta-data format, which is embedded within image files themselves (by cameras, etc).