Heading Background image to big

Hi there, I want to set a different Heading Background image for each of my tour pages i.e 1 day tour, 7 day tour etc. But the image is always to big for the box.
The box header size is fine. I have tried to make the picture many differnet sizes and use panarama images off the net. I want to lock the picture size to fill the box but what ever size i make it, it always over hangs. Thanks for your help in advance