Well, a more pertinent question would be how it is printed...?! Textile printing is quite specific and from simple foil transfers or inkjet printing to genuine silkscreen printing, different procedures apply. That and of course the requirements of the facility actually doing the printing/ transfer. If you haven't even thought of those things, then there is no point in even beginning to talk about the topic. I could tell you to make it all looking pretty in Photoshop and when you go to your printer it al lturns into smoke because he wants it vector based or at least needs a vextor based die cut line. So by all means, investigate the production process first, then we can tell you which tools to use.
The bags are being printed by a company called natural bag company based in dundee, uk. The are happy to use jpgs. I have done this process before and used photoshop but i can't remember the version - which will probably be different now anyway. I don't need help to use the product - i'm fine with photoshop - i just wanted advice on the best download to try. thanks