All you need to do for instant calendar fame is to head out and take some screenshots of your character in seasonal areas. Find some amazing winter, spring, summer, and fall-themed backdrops for your photo-shoot, and get shooting!
The screenies will be available for download each month, and will contain the character's name, guild name, and server.
Don't forget to remove your UI from the shot, using alt+z or option+z on a Mac. To take a shot if you use Windows, simply press Prt Scr, located up above the Delete key. If you are a Mac user, press F13. Screenshots should be stored in your World of Warcraft folder, in a sub-folder labeled Screenshots.
If you are using Vista, they may be stored somewhere tricky. While you may be able to find this file path, I had to actually search for it, as clicking through the folders along the path didn't work for me. C:\Users\%username%\AppData\Local\VirtualStore\Program Files\World of Warcraft\Screenshots.
If you'd like to increase your camera distance, try out an addon like Improved Camera. Now get creative, and get submitting!