When you upload the photos you have to do it in jpg.
After the section of the T-shirt Click customize product ---> image, and from there loaded the photo png, only by the shirt.
Actually FAA accepts files up to 30 megs. I happen to load files from 29,90 mega.
You can easily load files from cameras 24 mega Raw; jpg turned into 8 bits.
Also load files from scanning from negative that are even bigger.