It's very difficult to prevent the images from being stolen. You can take some basic measures, such as disabling right-click in the browser, but someone who knows technology can get around that in seconds. It'll stop less sophisticated users, however.
You can also download free image editing software, such as GIMP. It's not a Pulp Fiction reference; it stands for GNU Image Manipulation Program. It's similar to Photoshop, but free.
https://www.gimp.org/
GIMP is very powerful but it does have a steep learning curve. Thankfully it's pretty easy to use it to add a watermark.
Do a web search for "GIMP watermark image" and you will find all sorts of tutorials.