Background gradient won't gradually fade to white

I want a background gradient that starts as dark blue at the top, then fades to white. I changed Twenty Eleven’s background to white so the background would fade into that. I also put the background in to repeat horizontally but not vertically.
But no matter how many times I create a gradient background in Photoshop (where it looks okay), when I upload it to WordPress, it seems to fade to light blue, then stops and changes abruptly to white.
I’ve tried gradient images that were only 3-5 pixels wide as well as ones that were closer to the width of my browser, and both had the similar problems.
I feel like I must be missing something obvious, either in WordPress or Photoshop, but I can’t think what.
Also, is there a recommended length for gradient backgrounds like this?