There is an adage about a renowned sculptor who spent as much time working on the back of a sculpture (which would be eventually be standing against a wall). When asked why he would spend time working on something that nobody would see, he replied, "Someone will see."
Take an example of this beautiful landing page for the Moto X and how it looks when loading the background fails (which really happened to me, I didn't orchestrate this!)
So yes, if you are using white text make sure you have that area as dark in CSS. Do not rely on some image. Perhaps you might even want to gracefully degrade if the css is not available...