I'm working on a website that's already been designed by someone else. The designer has used a big image (900x700 100KB) which contains a big logo right across the top, then the background for two columns.

This image loads every time a page is loaded as it forms the basis for the website. What should I do with it to improve loading time?

I'm considering splitting it up into two or more images, especially the logo on the top. Does splitting up images like that decrease loading time in any significant way?

Thanks

-edit: Also, all the images are .jpg, would changing this to .gif or .png help anything?