A watermark is a logo or text inserted on a photo for the means of identification as well as a deterrent from someone copying your image.
The most common use for a watermark is to deter someone from copying your image. For those trying to prevent image theft, the watermark can be a good example of keeping honest people honest, but not necessarily the same can be said for the thieves. Depending on where the watermark is placed, one could simply crop it out of the picture. Ultimately, the watermark offers limited protection against theft unless you do a full-page watermark, which almost defeats the purpose of posting it.
—Just My Imagination DeZigns
