This is a really important one that I only touched on briefly in my Your Portfolio Repels Jobs article.
If you’re emailing your portfolio around, or even if it’s just on your website, name and watermark your images. Put your name, site URL and email address in the corner of every image so it’s noticeable but not annoying. NEVER cover up what’s in the image.
Also name it something helpful like Jon_Jones_Character_Artist_ManWithAxe.jpg. Keep your name in it, and on it.
This is the way to think about it: If this image were removed from my website and sent around at random and was removed from ALL context, how would someone that looked at it be able to find me?
Make it as easy as possible, and don’t assume that people will want to try very hard to find you. Even if they WERE willing to try really hard to find you, make it easy anyway. Take any advantage you can get, no matter how small.