When it comes to using existing media (artwork, video, audio, photos), it all depends on what you want to do with the finished product.
These tools can help you brainstorm images and outline and check your project.
This worksheet can help you figure out what kind of images you need.
You might want to begin your work using a storyboard to visuallize your content.
Make sure you can answer these questions before presenting or publishing your final project.
