Background card images should be 320x240, 16 or 24 bit BMPs. Most image editing/creation application will provide a way to export files in those formats. Remember that using smaller depth (i.e. 16 bit instead of 24 bit) will result in smaller image files. This isn't as much a concern with the PyPortal, especially if you have your files on an SD card.
Freesound is a source of free to use audio for various purposes. These are generally uploaded by community members and have varying quality.
Regardless where you get your sounds, they will likely have to be converted to work/fit on the PyPortal. See the guide on sound file conversion for details on how to convert audio files to this format.
The example shown uses the PyPortal's flash file system to store the adventure files. To more easily share your adventures or to handle very large ones, you can use a micro SD card. The code.py shown has the code already to use the card, and all you need to do it specifiy /sd (or a subdirectory) in the call to load_game.