As requested: Mini tutorial for GIMP. You can get GIMP at http://www.gimp.org
for FREE! I am using version 2.4 and running Windows XP (Though the tutorial should apply no matter what OS, as far as I know).
To make a single image showing the front and back of an ATC you will need to open a new document that is twice as wide as the original, but the same height as the original. For example, if your image is 200x300 you will need to open a new document that is 400x300. Do this by going to the main GIMP window and choosing FILE>NEW. A new window will pop up asking for the dimensions you would like the new image to be. After you have put in your desired dimensions, click OK.
Yay, now you have a new blank image!
On this new image window, go to FILE>OPEN AS LAYERS. A window will pop up. Navigate to where you have your ATCs saved. Choose one of your ATCs and click OPEN. Your ATC should now be centered on what was your blank image. You are going to choose your MOVE TOOL (it looks like a 4 sided arrow).
Click and drag your ATC to either the right or left.
On your image window, choose FILE>OPEN AS LAYERS again. Now choose your second ATC image. Using your move tool again, click and drag on your second ATC image to the opposite side as the first. You can fiddle with their placement using the move tool until you are happy with it.
Once you are happy with the final result, you are ready to save! On the image window, click FILE>SAVE AS. You can navigate to whatever folder you would like to save your new image in. Name your new image (eg "January_ATC.jpg") adding the extension (.JPG) is important! If you do not type the extension, GIMP will save it in it's native format (.XCF). This is good if you want to go back and edit this image later, but not good if you want to show it off to your friends.
That's it! Feel free to ask questions.