In brief
background-image can specify multiple images separated by commas. The first listed image is painted closest to the viewer, above subsequent layers.
Useful to know
background-image can specify multiple images separated by commas. The first listed image is painted closest to the viewer, above subsequent layers. [1]
For a gradient over a photograph, ordering matters: an opaque front layer can hide the photo. A background image also does not replace a semantic image with a suitable text alternative when it conveys essential content.
Sources
For further detail and to check the information: