From within the design editor, select anywhere on the design background.

Select the color box in the top toolbar that says Fill. 

In the left panel select a color from the template, Brand Colors, or use the color picker to select a new hue. Hex Codes can be entered for colors as well. 

Select anywhere on the design background.

Select the color box in the top toolbar that says Fill. 

In the left panel, across from Color Type, select one of the gradient boxes.

Gradient options are Vertical, Horizontal, and Radial.

Gradients allow for two color selections.

In the left panel select a color from the template, Brand Colors, or use the color picker to select a new hue. Hex Codes can be entered for colors as well. 

Select Graphics from the left panel.

Select Photo Area.

Select Full Page.

This will add a photo space over the background fill color.  The photo frame will appear as a gray space.

Select the text All Graphics. 

(Alternatively, selecting Graphics from the left panel will navigate to the same options.)

Pick from Stock Photos or My Uploads.

Drag the chosen photo onto the photo space, which will fill the entire page. 

Select the background on the design.

Select the Trash can icon in the upper right.

If there is an image placed in the photo area, two prompts will appear - Delete, and Remove Photo.  

Delete will remove the photo area and the image. 

Remove Photo will delete only the image, keeping the photo area.

For more visual guidance, please watch this short video demonstrating the process: