If you don't have photoshop or just don't want to bother with it, I have attached a word document with the images I used on my cube as well as the Gears logo in six different colors if you would rather use them.
To get started choose 6 images that you would like put on your cube. I chose 5 characters from Gears of War and the Gears logo. I initially went to the local comic book store with the thought that I would pick up a few comics and cut panels out to use. This will work, but doesn't allow you to manipulate the colors. it also adds a little to the cost of the project. Instead I went online and found the images that I wanted to use and downloaded them.
I opened them in photoshop to play with the color a little. Once the image is open in photoshop go to Image>Adjustments>Hue/Saturation
Make sure preview and colorize are checked. At this point play with the hue, saturation, and lightness until you get the color that you want.
Next you need to crop the image into a square. Again go to Image>Canvas Size
Here I adjusted the image to be around 8 inches by 8 inches. This image was then adjusted by going to Image>Image Size
I adjusted the cropped image to 2 inches by 2 inches. You may need to adjust this a little to compensate for the size of your cube. My cube measured 2.25 inches. These images were added to a word document and then printed.