Touch Develop does a really nice job of generating all of the images needed for a Windows 8 application that is being exported IF you provide a good imagine as part of the export. I put together around 90 images that can be used during the export process from Touch Develop to generate the images for your application. You can find these images here. If you’re building converter apps, weather apps and the like these images are a great place to start.
Additionally, I put together the following blog post and accompanying video if you want to build your own images. It’s pretty easy to create app art if you have PowerPoint.