Yep, figured that might have been it. Well, I've done a little drawing with a computer before, but I don't know that I can really speak for good techniques. However, I do think paint.NET is a good option for this, it has lots of good tools to use. Another option is GiMP, which I havent used before. Both of which are free, which is nice since photoshop or a professional drawing program costs a ton.
Paint.NET takes a little while to get used to, but it's very powerful once you get the hang of it.
One technical advise bit I can give, is that when you are trying to do something precise, make sure to zoom in to maybe 600-1200%, which is what I do. If you look very closely at my banner below with the brown plane flying over the town with a train, you may be able to notice that the back of the plane has been edited. It used to have symbols like the other planes. What I do with that kind of stuff is zoom in really far and try to copy/paste in places or color match and camouflage so it looks like it's meant to be there.