The people from french Simutrans community have written a detailed tutorial, full of sample images, about how to create an addon. From the png image to the final pak file, it covers almost all stages of the creation process. It’s specially recommend for newbies who want to develop their first object for Simutrans.

The original article is available in french, but if you want to translate it to your native language, put in contact with your wiki admin, get an account and translate it!