Profiles have a background hero image at the top. As the name suggests, that's the image in the background just behind the main image of the agent (for profiles) or company logo (for offices and companies). The image appears blurred or darkened, depending on the browser you use.
The background image is taken from the high resolution first images of the profile's featured listings. If the first image of a featured listing isn't high resolution, it won't be eligible. In the lounge you can edit featured listings by opening your main profile and selecting the "Featured" tab.
If you don't have any featured listings on your profile, don't worry. The website will default to choosing a background image from a random listing on your account.