A cover image is the picture that will sit at the top of your idea or challenge as the first thing that is displayed not too dissimilar to a cover image on social media. It is a great way to showcase the concept of what your idea or challenge is all about.
Having a cover image for a challenge increases the chances of it being noticed so make sure you choose a great one! Similarly with your idea, if your cover image represents the idea perfectly it is going to feel more coherent for the recipient of the idea.
As you can see here, the cover image is displayed at the front of your idea and challenge.