Please follow the steps below to apply Footer background image using the Prime Stylizer.
- Launch the Prime Stylizer by clicking on Settings Gear (SharePoint 2013/2016) or Site Actions (SharePoint 2010) and selecting the Prime Stylizer option.
Go to Footer section and in Background image select Add Image.
- Upload Background Image dialog will appear. Select the desired image and click the Apply button.
Set following variables if image is too small or large.
- Set image transparency to minimum to display complete image in footer background.
Footer with background image will be displayed as below.