Using a slogan as image in your logo is possible. But wouldn't it be better when that slogan was text? Easy to modify through the admin, but better yet, also indexed by Google, because it is text instead of an image.

Set a slogan text for each store from the admin at the store settings menu.

You can then place the slogan anywhere in the header.tpl template. In this VQMOD file it is simply placed under the logo. You can of course attach your own stylesheet to the slogan.