Leet me show in a different way. A picture speaks a 1000 words:
As you can see I replaced the original image and changed the size to "cover". It worked well...
But, when I changed the size to contain, the original image is still beneath the image I replaced it with. You can see this in screen shot below. how do I remove the original image?
Interesting... There's a number of potential explanations, one of them being that the new background is applied to a different element. If you can upload the .rsd project file (to sdrive or your own server) and get us a link we can take a looksy.