Use the standard jAlbum trick for handling theme images. Find the image you want to use as the theme image. Make sure it's large, like at least 2000px wide. Now add it to the project, and drag its thumbnail over to the Theme image box on the lower right. Now exclude it from the project - don't delete it, just exclude it. There's an icon for that along the top row (looks like a "do not enter" sign), but it's quickest just to hit the minus key on your keyboard.

jAlbum will scale that image, but not to the normal image bounds - you can set the maximum image width at Mercury > Title Bar > Theme Image > Width. (If it didn't scale it at all, you could end up with the page stuck with loading some 6000px monster, and it would take too long). It will use it for the theme image, but the image won't be included anywhere else - there won't be a slide page for it.

You can do this in almost any skin that uses theme images.

The maximum spread of the navigation icons is controlled by the maximum width of the thumbnail table - Mercury > Layout > Thumbnails > Layout > Max table width. So, they're always in the same place on every page, regardless of image sizes. I've seen users set that number very high, which spreads their thumbnails out all the way across a large monitor. Looks awful - the human eye can't scan a thumbnail table that wide. The default value of 1250px is chosen for a reason.