Thumbnail renders correctly, but the texture does not.

I'd have to see your file, or know more about what Blender version you're using, and how you're setting everything up. Would I be able to see your file? I might be able to answer.

email me: paul@cgcookie.com and I'll take a look! It *could* be a version thing. I'll see what's going on. I am beginning to suspect it is the size issue? Your texture renders at thumbnail size, or vise versa?

For anyone that encounters this issue in future: This was a bug in version 4.1. If you do everything correctly, open the file in 4.2 or later. You don't have to change anything, just hit render and you won't have this issue.