Did your tent come with a rain fly? Usually that does a pretty good job of keeping the rain off, so you don't have to rig up a tarp over it. I also like to spray the outside of the rain fly and bottom edge of the tent with Scotchgard Outdoor water shield for added water repellant.

Also, I noticed you don't seem to have anything under your tent. That is where I normally put a tarp. The ground is where you loose most of your heat, so the more insulation you have between you and the ground, the better. It also serves double duty of keeping the bottom of the tent clean and protected from rips/holes/tears which can allow water (or critters) to get inside.