Have you tried any built-in sprays, or only custom?
If it's happening with the custom sprays and not the built-in, than it's something wrong with the images you're using (Could be something simple like the colour depth of your image).
If it happens with ALL images, than you have a problem that may require Valve's tech support.
Are you sure it isn't an image problem?
I remember I had this cool lookin spray a while back but I could never see it
I changed sprays and now it works fine