Once upon a time, making commissions had few implications in terms of illegal usage or distribution of artwork. But like with the previous two aspects of commissions I wrote about (Pricing and Design Work), the question of who owns the artwork you produce has been brought to the forefront thanks to reproduction and distribution technologies becoming easy to use and ubiquitous. When photocopied fanzines or manual desktop publishing were the most you had to worry about, amateur artists really had no incentive to worry or know about the question of rights ownership, especially when it came to making custom art for a few dollars. Nowadays, digital photography combined with a means of distribution exists in hand-held devices even children aged in the single digits are experts at using.
Technology will just keep making it easier for art to be copied, deconstructed, remixed, shared and consumed. At a time when anything can be the 'next Internet sensation' and large monetary interests have invaded the channels for digital distribution, the question of who owns the art you make has got to be part of your strategy as an artist, amateur or professional.
Whether you subscribe to the Lawrence-Lessig-proposed view of Remix Culture and think all content should be freely shared, assimilated and re-processed making society richer, or the pre-Internet one of strict rights ownership and control, there are a few things to have in mind:
1) Content posted on the Internet is de-facto shared with everybody. You can ask, plead, beg or threaten your audience not to share, copy, alter or pretend they own your work, but while technology makes this a no-cost proposition, it will be done. Ask the Music Industry how they're doing in their futile attempt to thwart unauthorized distribution of non-free music.
2) Creation of a work of art grants the creator all rights to that work, unless he explicitly trades, sells or gives them away. Doing commission work where the rights are not explicitly mentioned in a working contract keeps the rights with the creator, BUT an uninformed customer may think that once you sell them a piece of board with your doodles on it it is his own to do as he pleases.
Managing rights ownership with commissioned art is tricky because how much of a concept you own is usually not well defined. If a person asks you to draw a character from a vague description and you end up designing it, you own that design and all associated rights, and the commissioner can argue he is a co-creator. It used to be that most designs commissioned by individuals wouldn't see the light of day, but technology is bringing us to a world in which every piece of content will be publicly accessible from the moment of creation. And if it is good, someone may like it enough to want to profit from it. Making it clear from the moment of creation that you own such rights is the only sane and easy proposition to make sure your work is not lost to someone whose lawyering pockets are deeper than yours.
I suggest you make up your mind about who will own the rights of what you create. You can share your art on Facebook only to your friends, or make everything you post publicly at DeviantArt Creative Commons, or make a 'fine print' Terms of Service contract where you state who owns what for every commissioner to read when they hire you. How freely owned you want your content to be is a decision you don't want others to make.
Here's another doodle from another commission I got in the making. Tarot-inspired.