In the case that Zazzle won't allow you to use your own design (it looks GREAT by the way, nice idea!) and you need to use iron-on transfers, here are a few tips. You can get a decent sized pack at a reasonable price, I made them for our last trip specifically for the Pirate and Princess Party since it was August and we didn't want to wear costumes. I figured out their final cost to be about $3-4 per shirt. I've also made lots of other iron-on shirts (Dharma shirts for a Lost finale party for one!), at least a dozen or so. Here's what I've learned:
-Unless you buy transfers that specify "for dark colors" only use the transfers on light colored shirts. I chose some pastels and a light grey because I don't like plain white t shirts. You can usually find reasonably priced tee's at a craft store (like AC Moore or Michael's) and even places like Target. I've purchased them for $2 a pop at AC Moore if you hit the right sale. Even if your design uses all black, the transfer itself itself will show on a dark shirt. They are more "translucent" than "transparent", so you will see all the "blank" space in the middle of the design. So in your case, unless you want to sit and trim all the background space out of each design, it could look sort of funny, kind of cloudy looking.
-Wash the shirts before you do the transfers. If you put on the transfer before washing and it shrinks a little during the first wash, it will stretch the transfer. Usually people want the shirts JUST for the trip, but you might as well get some wear out of it afterward or even for a next trip.
-On the same note, washing the shirts inside out after you've ironed on the design will help prolong the life of it. This and the other tips included in the instructions are important to follow like not washing them in hot water, as it will also crack the design.
-It's nice to personalize each shirt for each person. Maybe considering using a main design (like the one you posted) but also add something for each person. Could be their name, favorite character, lots of things. In our case, no one had the same shirt design or color. It was nice to have a "group" shirt without all looking identical.
-When you print the design, it needs to be printed IN REVERSE if there are any words on it. Otherwise, you will have backwards text. Not good! Unless the design is symmetrical, this is important. I've wasted a few transfers because of this! They are reasonable but not cheap. Ended up needing to buy a whole extra pack for just ONE transfer after I ran out after too many boo-boos. Also be sure to know which side your printer prints on (bottom or top of sheet in the loading tray) as you can end up printing the design on the wrong side of the transfer, wasting a transfer AND ink!
-Be sure you print to take up the entire 8.5x11 space of the sheet, if you want the design to be large. This is easy to do with any basic photo editing or printing software. You can also utilize the whole sheet by splitting it with two designs and cutting them apart.
-As mentioned above about the translucent but not transparent texture of the sheets, you'll want to be sure to trim any excess around the edges of the design. It can really improve the overall look. It can be cumbersome if the design has a lot of outside detail but really does make a difference.
Zazzle IS great and I've ordered other shirts from them via disneyshopping.com before, so if you go that route you'll be pleased. It may, however, be more budget friendly to do the transfers, especially if you can get the t shirts at a good price. Good luck!