First: You pasted the image onto the layout of whatever was the current slide, not to the master itself. Depending on what you want to do, this may or may not be ok. If you want the image to appear on ALL slide, as I expect you do, choose View | Slide Master then scroll to the top slide in the thumbnail pane, the one that all the other thumbnails are slightly indented under. THAT is the master; anything pasted there should appear on ALL slides.
Next: With the image pasted onto the slide's layout, it's probably appearing on the slide, but you've got other stuff covering it up, ie your own image(s) on the slide. Select and delete anything that's not supposed to be part of every slide and delete it. The image you pasted onto the layout should then be visible.