We are here at the //publish/ event for Toronto and one of the goals is to polish up apps and get them submitted. We are lucky enough to have Kevin Carey our designer at RedBit be here to help developers polish apps with design suggestions, creating visual assets and just overall design suggestions of apps.
Not everyone has a designer they can leverage, so here are a few resources for helping add some visual design to apps. Here are a few links and resources for graphics and also sound effects which are great (and required) for games.
If you have some other links, please go ahead and share in the comments and I can add.