It could be a simple fix.
First off, I would avoid NP's manuscript editor. It will screw up your file and cause more headaches than you care to deal with. That said, take a look at these.
You need to make sure the name of your cover file is exact.
What I mean is, CAPS matter as does your use of .jpg or .jpeg
Also, I see that your image is 1500x2000
That might be your issue. Golden ratio for a books cover is 1333x2000 (1600x2400) You might want to try resizing it; see how that works for you.
One more thing, did you verify your epub before you uploaded?
I don't see in your code where your books cover would be listed in the stylesheet.
Just a thought . . .