Here at Showcase, we find Adobe InDesign to be one of the most useful pieces of software to design Showcase presentation background slide images with. InDesign has a good balance of typesetting and image-based capabilities, including robust vector support.
If you're thinking of using InDesign to create your Showcase slides, we've made it one step easier for you with some IDML templates in our six default ratios and orientations.
We've added the Showcase controls (the circular buttons in the corners) on their own transparent, non-printing layer. That way you can design around them and then export a clean image when you're done! These IDML templates are also available from the "New Presentation" screen. Click the links below for a direct download.