We don't disagree often, but I always supported Paintshed. The concept is brilliant, but the scope never widened much. I appreciated that it gave users a easy method to make their own liveries, to allow them to get more enjoyment from the game, and have some personalized content.
I don't know what popular graphics programs work in Mac, but that's the first thing you'll need. There are other tools that you can use to make it simpler, but I don't think they are built with Mac in mind. You'll need to isolate and convert the texture file(s) to a form you can adjust and then make the changes you want before saving and importing back into Trainz. It helps to clone the piece of content you want to work on. I usually just "Edit in Explorer" then copy and paste it on my desktop where I do all the work. I then change the config file as needed, and bring it back into Trainz as a new asset.
