Hi,
I’m having this problem for the first time. I uploaded new key images for one of my packages and pressed preview. Even though all images are different, since I replaced the old with the new images, they are not changing in the preview. I still see the old images in the preview. Since I can’t see what the preview is like I didn’t submit the update yet.
If you could let me know what’s causing this or how to fix this issue I’d be delighted.
Thanks! 
Gold is the new images.
Blue is the old images.

Now, the small and the big key images are blank and the icon is still the old blue one. I e-mailed Unity just now and waiting for an answer. Here’s a screenshot:
This problem exists for about 2 weeks now, see here. I also sent a support request to Unity a week ago and their repsonse, which I received yesterday, is they’re working on it.
Thanks Peter! I’ll keep an eye on your thread.
THIS ISSUE IS RESOLVED! Thanks to the awesome Unity team, I got an e-mail from Ben Stoneman today and he told me they fixsed it. The Asset Store Kit is upadted. I was told to try again one more time by reuploading the images before previewing and it looks like everything is working.
Thanks guys!