In the Sparkle Admin panel, you have a choice of our pre-made graphics or you can upload your own. Elements that you can customise: Banner image (1100 × 376 pixels, JPG, 72DPI, RGB), square profile image (400×400 pixels, JPG, 72DPI, RGB), map background (2000 × 1200pixels, JPG, 72DPI, RGB), room graphics (minimum 400×400 pixels, transparent background PNG).
To make a graphic transparent you can use an online service like http://remove.bg to cut out your images.
Depending on your level of skill, you may want to wrangle a friend who is a graphic or multimedia designer to help bring your vision to life.