For someone getting started in writing for RPGs, I don’t think commissioning art is the right place to start. You simply do not have enough data to correctly forecast future sales, for a start.
Free art on the other hand is a much more tempting proposition.
I want to discuss two options. Public Domain and Homebrew.
There are many repositories of public domain art that you can use for your project. They fall into two camps, either out of copyright art such as 18th Century engravings or modern creations that have been put into the public domain by the creator.
There are a couple of problems with the first option. When you are using a library of old art, you are limited to the art that is available. The artists are long dead, there will be no more art and if what is available doesn’t work for you then tough!
You will also find some of the best public domain art turning up again and again in rpg supplements simply because the art that is suitable for adventures is an even smaller subset, so we end up all wanting the same images.
Tyler A. Thompson of Sad Fishe Games lamented recently “really wish these 1800’s era engravers made about 20 times as much stuff as they did” and that is the problem in a nutshell.
Homebrewing your own art is another option.
I am not assuming you are suddenly going to become a super talented digital artist. What you can do is take a new piece of public domain art, or even an old piece that isn’t quite what you wanted and edit it.
This is a one page dungeon I wrote for Dissident Whispers.
The naked lady on the left originally featured in H G Wells War of the Worlds. The image came from OldBookIllustrations.com and looked like this…
Removing the frame, applying a gradient mask and then radial fill has lifted the image and blended it with the page background.
To answer Tyler A. Thompson’s lament, you can find tutorials online for PhotoShop, GIMP or Paint.net on how to turn a photograph into a woodcut or engraving. Once you have learned that technique you can then use anything from your own photos to any public domain image to create images that all fit together, look like a coherent whole. In addition, you will have images that no one else has.
Images used well can add a lot of value to an otherwise ‘wall of text’ document. It can add flavour and help the GM visualise what you intended.
Once you have art, that opens a whole new can of worms, page layout!