Changing the default banner image on a workflow brings life to the page and can be a great way to help constituents better understand what a particular process is for.
To change your banner image, first visit the Start page for the workflow. You can find the link in the "Workflows" section, on the "View" tab for a specific workflow. Click the "copy" symbol (two squared) from the "Use workflow" option where it says "Share the link to start a process" and paste the link into a new tab on your browser.
Once on the Start page, hover over the default banner image and click the text that says "Change banner image" in the bottom right corner of the banner.
Select an image to upload and you're good to go! Please note that the system greys out and darkens the image so that the white text is visible, so you may find that contrast and visibility is limited.