Does your image meet the size requirements specified on the upload page?
"Your header image must be under 4096 Kb. Images larger than 1800x720 pixels will be scaled automatically."
If not, you may need to use a photo editing program to make changes to the size so it will work. Otherwise it will usually end up being distorted.
Here's a link to a freeware program that should work if you don't have one:
I hope this is helpful!
If you start by creating a blank image that is exactly 1800 pixels wide by 720 pixels in height, then cut and paste your image inside of that and center it, it should come out fine and be centered on the page when you upload it. Just make sure your image is smaller than the 1800x720 blank. You can see how mine came out as an example. Mine says it is 3.71 Mb in size.