How to watermark images, and whether it is worth it
Put the mark across the middle of the image at around 40–60% opacity, not tucked in a corner. A corner watermark is cropped off in two seconds; one that crosses the subject cannot be removed without damaging what somebody came to steal. No watermark is unremovable — the goal is to make removal more effort than it is worth.
Corner versus across
A small mark in the corner is the usual choice and the weakest one. Cropping removes it entirely and costs nothing, because the corner is rarely part of the subject.
A larger, fainter mark across the centre cannot be cropped away without losing the image. At 40–60% opacity it stays readable while leaving the photograph viewable, which is the balance you want: a deterrent rather than a defacement.
When not to watermark
A watermark costs you something. It makes the image worse to look at, which matters if the image is doing promotional work, and some platforms down-rank visibly watermarked uploads.
For portfolio images at screen resolution, consider skipping it: the file is too small to print or resell usefully, and the mark reduces its persuasive power. Watermark the high-resolution files you send to clients for approval, not the small ones you publish.
What a watermark is actually for
Attribution more than protection. It tells an honest viewer who made the image when it has been reposted without a caption, and that is genuine value even though a determined thief will remove it.