I tried to resize the cover image to make it larger and it didn't work. It was rejected. I did a new cover at the right size for the site and it uploaded just fine.
I set the size in the program for the image to the right size and then did the cover. It may be that you have to start with the right size, rather than try to resize the image.
I doubt this is much of a help[.
Do you use PAINT? You can download and resize pictures from there. You need to do a minimum of 900 pixels for the first number and at least the same for the other minimum. Should work for you then! NO matter what do at least 900 pixels and you shouldn't have any problems.