Usually it's via PayPal, and unless it's a very big company you should ask for portions of the payment at different key milestones. Or at the very least only send the high res image after they pay you, and only send them thumbnail versions of the WIPs (something they can't realisticly use commercially in case they decide to run)
This way they can't bail on you at the end, and they can't easily request infinite revisions.