Do yourself a favor. Somewhere on your sketch write this simple text line: "© [current date or year] by [your name]." According to the Copyright Basics circular from the U. S. Copyright Office <<http://www.copyright.gov/
In my examples, I make it a habit to always write “© 2014 Wesley E. Douglas” along the edge of my sketches. The reason I am suggesting this is because it is not the responsibility of anyone who views, shares, repins, retweets, or reposts your image to make sure you are properly credited. That responsibility falls on your shoulders. And by writing this simple line directly on your artwork it will be less likely to get separated from the sketch than adding it to the comments box.
This simple line of text will actually solve a few common issues with posting images online:
1. Your sketch will always remind people that you are its rightful owner. Regardless of how strongly you feel about whether your sketch deserves to have your signature attached to it,
this is not the time to be shy.
2. When a media outlet wants to use your image for an article they are working on, having some kind of identification on your artwork will make it easier for them to contact you for permission to use your image.
3. When you are staring at a blank page in your sketchbook, adding this simple signature makes the perfect first mark on your page.