Thanks for the comments.
The first shirt I made that went through the drier twice is starting to crack.
I found a forum on making iron on transfers and read a lot of useful tips. Unfortunently it has come 3 shirts too late. The last one I made was the gecko shirt which looks to have adhered much better then the other attempts.
Here are the others I've made.
I'm going to hand wash these tees and air dry them too see how long they last.