What image size should be used for mobile website?

Our target mobile size is 320 x 480. Our mobile break point is 725.

My header image will fill the entire width of the mobile screen.

What width should the image be?

A) 725. This way, for those using phones with a screen narrower than 725, since the code will shrink the image (set at 100% width), at least the image has enough pixels to look good.

B) 320. This way, for those using phones with a screen wider than 320, since the image will be enlarged by code, this might result in crappy image quality.

I’ve been doing 725 in that past, but wanted to make sure this is the right way to go.

Appreciate your feedback!