The problem is that this application will serve infinite users with infinite albums, so while img_id is unique and sequential, it doesn't gaurantee that any two images belong to the same person or album.
Re^2: Building an index for next/last in a photo album.
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You could easily keep the current album seq_id value in another table and reference it when adding new images. As I said, it may be a hassle modifying your database, but it may be worthwhile in the long run as it more easily does what you want than the other options.
Note: This may be a good "teachable moment" as educators like to call them.
Perhaps the most important thing in designing an application that uses a database is to know thoroughly what you want from the application before you even begin designing the database. Too often, people design a database based on a less than thorough examination of the application requirements and end up having to redo it (the database.) :)