Hi Sue, I don’t have a plugin to recommend but, as you may be aware, it’s virtually impossible to prevent someone from grabbing a photo if they are determined to do so. For one of my projects I link to a lot of articles and photographers’ galleries, and I like to pick up an image to use as a thumbnail. So it’s become a game to me and I have yet to lose. Also, ironically Google Images is quite proficient at extracting images from protected galleries, so people can pick them up from a search.
If you simply want to slow people down and/or discourage them from copying the image, there are a lot of plugins to do that. If the images are really desirable you may want to consider watermarking them. I installed Image Watermark by dFactory for two of my clients and they’ve both been happy with it, and there are other watermark plugins as well.
You can also edit the image’s metadata so that copyright and credits travel with it. Facebook displays the information with uploaded, and Google Images just started doing the same in September. But that’s not foolproof either as the metadata is easy to remove. In fact, many image optimization plugins automatically remove metadata as part of their size reduction. If protecting images is critical, you might look at services like Imatag.
Finally, I don’t know anything about the images and how they’re used, but in a couple of cases I’ve pointed out to clients that making images shareable can actually be to their benefit. That’s another case for a watermark that will travel with the photo when it’s posted to Facebook or Pinterest.