Carsten Haitzler (firstname.lastname@example.org) took the time to produce very nice cross-referenced documentation for Imlib2, but for some reason (size considerations most probably), it is not included in the Imlib2 source tarball1. You can have it here .
Most functions of the Imlib2 API have corresponding, homonymic commands in adesklets. See Programming adesklets, for specific details on the transition from direct Imlib2 programming in C to adesklets.
 Although it can be quite easily regenerated using doxygen.