There is a practical consideration for inclusion of your name right in the posted image.
If the image is interesting in any way, there is a good chance that someone will copy that image and repost it without any ill intentions to another board or gallery.
There is also a remote chance that someone else will find that second copy of the image alluring and would like to obtain legally a copy of it.
If it were a physical print, the artist could include his name and contact info on the back of the print, but if the digital version doesn't show artist's name, there is no way that the interested party could find and contact the image maker. However, if the name is right on the image, it should be easy to locate the image maker (unless that second poster finds the signature or watermark intrusive and removes it prior to his posting).