Tip: Many websites are 930px wide or more, but on phones (with their 320px screens) that's only OK if content is being "shrunk"; if you're using a mobile-optimized version then the regular banner won't fit there. However, YWD Designer™ allows you to specify a separate banner image for mobiles.
Create a smaller banner, just
for iPhone/BlackBerry/Android devices
The banner image should be 320 pixels wide (the width of the mobiles' viewport) and not very tall.
Upload the image to the YWD™ Asset manager, just like any other image.
Tell YWD™ to use this banner
for the mobile-optimized version
Launch YWD Designer™ and go to Mobile setings / Smartphones / Banner & content. Select the banner image you've created.
The banner image is applied: