Your best bet is to write it up as a contract which details their usage rights but maintains your ownership of the images. You might also wish to be credited when they use the images.
Just make sure everyone is on the same page before you start, and no one should have any problems.
I believe there are sample photographer contracts online all over the place...but maybe someone more law literate than me can point you to one.
M3, M6, 50cron, 50nokton, 90hexar, 35biogon, 21skopar.