Both images stretch to the full canvas. Transparent PNG pixels stay open so the shopper can design through them. Overlay sits on top; background sits behind the print area.
Background image (fills canvas)
PNG preferred. Transparent areas show the page color and stay editable. Do not include print/safe dotted lines in this file.
Overlay image (fills canvas)
Frame or product PNG on top of the design. Transparent pixels are the editable window. Do not include print/safe dotted lines in this file.
Optional extra guide image
Also fills the whole canvas. Skip this unless you have a separate visual aid — the editor already draws print and safe guides.