If you place any images on the internet, including portfolio sites, blogs, Facebook, websites etc, do you watermark them? If not, pop onto Pinterest and search 'illustration'..... there are thousands of images on there, most of which are not watermarked.
How difficult is it for someone to use an illustration from Pinterest or any other site for a website header, Facebook meme (you know those sweet little 'have a nice day' pictures people post up with a load of inspiring or humorous text), or to use the image for some sort of online advert / promotion...... I'll tell you how difficult it is.... a big fat zero in the score of difficult.
What are the chances you know where your illustrations are being used? A very slim chance unless you are constantly doing a reverse image search.
Artists, illustrators, designers and photographers are finding their images on sites including Pinterest, available for free sharing and ultimately ending up as free advertising for businesses. In some cases people are stripping the metadata, cropping away watermarks, erasing signatures etc, just to make it look like the images on their site or advert are legitimately licensed and paid for.
Most portfolio sites offer the option for 'pinning' (adding the image to someone's Pinterest pages... online scrapbooking type idea). What happens when you remove that image from the portfolio site? The image stays on Pinterest without the 'source' website showing your illustration.... ripe for becoming an 'orphan work' if someone chooses to remove your metadata, especially if you don't have a watermark ACROSS the image.
If you are concerned about the prospect of your work ending up an 'orphan work', watermark ALL images you put on the internet. Don't think that putting low resolution images will stop your work being used without your permission.... Consider this, how big a resolution do people need for a meme on Facebook? How big a resolution do people need for a website banner, background or advert?
Yes, having your work on Facebook, Pinterest, Blogs etc DOES get you exposure if you're a commercial artist, illustrator, photographer, designer etc...... but only if YOUR name is still attached to YOUR images.
If you think this only applies to commercial artists and photographers, think again. How would you feel if your photos were used for a political party with whom you had no affiliation? Or for a type of business that you didn't like because of their particular business ethics? Don't you think you deserve to know when your photos are being used by someone for profit, advertising or promotion? If so, watermark them... it's not hard and there are plenty of tutorials on Youtube!