Any graphics editing program such as GIMP or PaintNet, both of which are free; I've also heard of IrfanView (I think that's the name) which is also free. Personally I use Serif PhotoPlus which I bought at a reasonable price as part of an integrated suite including DrawPlus, PagePlus and WebPlus and I have used all of these now for some years.
There is no program that you can easily understand for creating content such as locos; the only way whether using GMax or Blender (the two most popular) is to start small by making, for example, a building and progressing step by step. Both can be learned - I use GMax and Trainz Asset Creation Studio (both free) and at the age of 81 I've still more to learn after five years. Don't rush things - that way lie frustration and disappointment. But whichever you use, don't hesitate to ask for help - I've found so many friends who are willing to help through these forums (Content Creation is the one to use).
Ray
Ray
