Comment by scaturr

Watermarking by urls or existing images would leave the non-watermarked image open - you are correct. This case is still fine for most users, but watermark.js also allows the use of file inputs. You can grab an image from a file input, then watermark, then upload it. This would leave the non-watermarked image closed off to anyone else. This would be handy if you need to generate watermarks in a CMS backend or some other administrative tool