As a photographer and scrapbooker, I am often asked where people can go in Iwakuni to print pictures. Now, there are many places you can go, but the one I like to use is Kitamura, in downtown Iwakuni.
Here is a link to the directions from the north gate of base: Directions. Essentially, if you know how to get to the Iwakuni City Hall (below), then you are a block away from Kitamura... at this spot in the intersection, turn right and drive until you see 7-11 on the right. Kitamura is the building to the right of the 7-11. Side note: the flower shop across the street is a lot of fun.
Once inside, you will see photo printing kiosks at the center of the store. Here are the prices as of May 2014:
There is an English option; click the Language icon at the top right of the screen. And you can use any number of media: memory cards, USB drives, iPhones, USB ports, CDs... the kiosks allow for lots of ways to upload your photos. One tricky thing at the end of your order: The kiosks offers to backup your photos for you. If you do not want this, you want to click the green "Exit without Ordering" button. Your order will go through, you just aren;t ordering the backup service. Be sure to grab the receipts that are printed and take one of them right to the cashier.
While I have never printed more than a few photos at a time here, I have never had to wait more than 10 minutes for my prints to be ready. They also have new and used cameras and lenses for sale, as well as other photography and scrapbooking/photo album goods. Customer service in English is a bit limited, but I have rarely had trouble communicating with the staff. If there is a word they do not know, like "warranty" they quickly jump on their smart phones to translate the word.