Images are an important visual component to your HeroX page and project or competition as a whole. Generally, your image should use little to no text on it. This will allow you to use the same image if you wish to run an advertising campaign on Facebook. Facebook is very stringent on text not covering more than 20% of the image. If you go over this 20% ratio, your ad will perform poorly.
In addition to little or no next, you want an image that visually conveys the goal or subject matter of your challenge. If you have a project to save domesticated animals, use an image that contains cats and dogs. If it's a coding competition, use an image that contains lines of code or a person working at a computer.
Lastly, make sure you're legally allowed to use the image. There are some great sites (such as Pixabay) where you can find great images with free licenses. If you're looking for something more specific or higher quality, use a paid service such as iStock to purchase an image.
Below are a few example images from competitions that were ran on HeroX.com. You can also look at some of the featured pages in the Explore section for additional inspiration.