That depends, inkpad.
In my line of work, whenever I do post a major work image or client's work project image, I'd add a copyright watermark. This does not guarantee that others will not use it. You'll never know, anyway. But it may come in handy oneday - settle origin disputes, source of image, or at worse, plagiarism lawsuits.
Sometimes the adage rings true - Whatever you post on line is open to public domain. A disadvantage to artists. Which is why it is a good idea to post a reduced size, low-res image online . Never post a hi-res, full size.
In other forums, most especially 3d forums, a copyright watermark is strictly required on ALL submissions before an image is approved for viewing.