Login to the Partica dashboard, then select the title you wish to customise.
Select the Custom CSS Editor tab:
After the lines of default CSS values, create a new line and paste in the following code, replacing the URL with your hosted image file.
Click Save and Publish once complete.
The image should be high resolution, and at least 1920 pixels in width by 1080px high. To ensure the flipbook loads fast, the image size should not exceed 2MB in file size.
The image will fill the available background space, however it will not stretch or warp.
It is important that you do not set a default colour value in the Background CSS in the Viewer Settings page, as this will overwrite the CSS you set in the Custom CSS Editor tab: