I am trying to create mobile friendly versions of websites, but can't quite work out the relationship between screen resolution and displayed images.

I have a T-Mobile pulse mini, which has a screen resolution of 240 x 320 pixels and is 2.8 inches. My problem is, I designed a page with a logo 800 pixels wide, yet it fits exactly in the Android web browser.

How can an image 800 pixels wide, fit in to a screen 240 pixels wide? The zoom indicated that it is zoomed out, so I assume it is actual size.

I just want to know how wide to make my header image based on knowing what the resolution is, is their a formula? I have data on columns, maximum image etc, but none is the 800 that would make sense.