Did you find the manual in the documentation of the official Gimp site (
http://www.gimp.org/docs/)? Right below that is a link to lots of tutorials covering a whole range of concepts.
When I started with it, back in about 1998(!) I recall working through a version of the
floating logo tutorial a few times, experimenting with different tweaks. That opened up two essential concepts - layers and masks - and also introduced me to the curves tool. Even though it doesn't directly address working on photos, it might be a good starting point.
Wulf