Your photos are your product, so protect them
What this fixes: Photos used by others · No watermark · Stock photos · Weak brand
Tick these off as you go. This page remembers where you got to, so you can come back to it another day.
Photos of finished water features get lifted constantly, and usually by competitors who needed a portfolio in a hurry. It is worth ten minutes of prevention on work that took you three weeks.
Watermark, but lightly
A small logo in a lower corner, semi transparent. Not a giant logo across the middle, which ruins the photo and makes you look worried.
Apply it to gallery and project images, not to every thumbnail.
If a customer or a supplier wants a clean copy, send one directly. The watermark is for the public web.
Name the files before you upload
koi-pond-your town-finished.jpg rather than IMG_4471.jpg. It helps your own search visibility, and it makes a lifted photo traceable back to you.
Metadata
Most phones and cameras write your details into the file if you set them up once. It survives a download and it is useful if you ever need to prove a photo is yours.
If you find your work on someone else's site
Screenshot it with the URL and the date visible. Email them once, politely, and ask them to remove it. Most do immediately, because it was usually a web designer grabbing images rather than the contractor. If they do not, a formal takedown notice to their host almost always works.
Do not go the other way
Never put a photo on your website that is not your own work, not even temporarily, not even as a placeholder. It is the single fastest way to lose trust when somebody recognises it, and in this trade people do recognise it.