Using Image/GD functions to transform images stored in database

I'm new to the GD/Image functions with PHP, and I'm trying to use them to manipulate jpeg images that I've stored in a MySQL database (specifically, creating thumbnails). The thing I can't tell from reading the documentation is how to use these image functions to operate on image data -- it looks like you get a GD image resource in each case by specifying a file name, not by passing image data directly. While I can see how that's convenient for the common case, I can't figure out how to make this work without writing the image out to a file (and since I'm already taking a bit of a performance hit by reading the image out a database, that seems like the wrong thing to do). Can anyone elaborate? Thanks, Weston