New Sessions include a default cover image. You can replace it with something more relevant to your topic.
Image size requirements:
Minimum width: 600px
Recommended size: 1160px wide by 330px high (or taller)
Steps:
Open the Session in Prepare mode;
Click Change image below the Session's title;
Click Upload image and select a file from your computer;
Drag the image to reposition it;
Add a descriptive caption for screen readers;
Click Done to save.
Where to find images
Adobe Stock (account required)
Pexels (free)
Pixabay (free)
Google Images
Whichever source you use, make sure the image meets the minimum size requirements above.