I use the photo editing website Picmonkey.com. If you use the design element off that sight, you can add pictures you want to a transparent background. If you find pictures of the Fantasy and the fab five with transparent backgrounds, it is really easy. If not, you can add regular photos and erase the background you don't want. Picmonkey also allows you to add text.
Once you've made your design, you save it to your computer. You can then upload your design to a custom printing site. I like to use Cafepress.com and Customizedgirl.com. Cafepress is always running specials on their items, usually for 20% off or more. Customizedgirl has a bigger selection of clothing that can be personalized including more kids shirts and lots of colors.