Convert jpeg to a 24-bit bitma[

Hi.
I need to save a jpg as a bmp but it has to be a 24 bit colour depth.
It's going to be the new splash screen on my wife's TomTom so it has to be 480 x 272 and a 24 bit bitmap apparently.
I've tried using iPhoto to save as a bmp and resized using Preview. But iPhoto doesn't give me any option to choose the colour depth.
I'd like to use either iPhoto to do this or perhaps some Freeware.
Can anyone help?
I have tried but I think iPhoto uses 32-bit colour depth so my image doesn't appear on the TomTom, just a blank screen.
Thanks,
Mark