Right-click a page in the HMIEditor page manager and open Page Properties. Configure resolution, home page, background color and transparency. Use Drawing → Image to select a local file. The manual lists jpg, jpeg, bmp, png and gif formats.
Match the background image aspect ratio to the page resolution to avoid compression or distortion. Changing the editor zoom helps design the page; it is not a guarantee of automatic reflow on every phone screen. After deployment, check text, dimensions and controls in the actual phone, tablet and PC browsers.