The linear timing function will animate the defined properties linearly. For the background-image it seems to have this fade/resize effect while changing the frames of you animation (not sure if it is standard behavior, I would go with @Chukie B's approach).
If you use the steps function, it will animate discretely. See the timing function documentation on MDN for more detail. For you case, do like this:
-webkit-animation-timing-function: steps(1,end);
animation-timing-function: steps(1,end);
See this jsFiddle.
I'm not sure if it is standard behavior either, but when you say that there will be only one step, it allows you to change the starting point in the @keyframes section. This way you can define each frame of you animation.