if I share an album using public website, can I prevent people downloading images

If you mean stopping people who know the URL to the album, yuou can't really stop them. One trick some people use is to place a transparent image over the actiaual image - this prevents control-clicking and choosing to download/save the image: but this won't stop people taking a screenshot. Professional libraries often watermark an image but then of course that's apparent to everyone, and only is useful if you want to allow people to use the image by paying.

Anyone who knows the URL can download from the shared album. Tell the URL only people you can trust to keep the URL secret.

Roger, the trick with the transparent image as a cover is great, but I do not see a way to cover the images in a shared album - as the page is created by the Photos.app and we can just provide the images to share.

That trick isn't of much value anyway as you can't stop screenshots.