Go to Content > Surveys, pages, and forms.

Select the survey, page, or form you want to add a background image to.

Select the block you want to add a background image to. This means you should select the background for the whole page or form.

In the left-side menu, select the Styles tab, and expand the Background images menu.
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In the Background layers drop-down menu, select the background layer you want your image to appear in.

Select Choose image to open the Image manager to add the image you want to use.

Select OK.
​To delete the image, select Remove image.