Best practice for chapter heading images

I'm sorry if this is a question that is often asked, I tried searching but didn't get very far.

The book I'm formatting for Kindle has chapter heading images. What is the best practice these days? Is it safe to use a gif image with transparent areas?
What resolution should such an image be? What is a recommended image size (especially the width)? I'd like the image to fill most of the width of the page.

Thanks!