Wow! Great color combo and theme you've got going.
We see so much cool wall art on this thread. It depends on your budget, but I think go with what you like to look at! I'm a habitual collector of old magazines and posters of the cars and motorcycles I've owned over the years, so I gathered a bunch together, measured them and bought black frames at Wal-Mart to put them in. I arranged them on my walls starting with the oldest on the left wall and working my way around to present day vehicles. It's easy to start covering wall space pretty fast. Amazon and eBay have a huge assortment of posters - automotive, racing, movie, beautiful scenery, etc.
I also like NEON. Either neon clocks or signs will ad a nice glow to your garage.
