You can't stop people downloading your images from the internet because your files are automatically downloaded to the visitor's computer in order for them to be able to view the page that the image is on. Once the image is on their computer you can't stop them keeping a copy.
What you can do is to make it obvious that the image is yours and has been taken from you either with or without your permission. One way to do this is to place a copyright notice into the image in such a way that it does not interfere significantly with the image but makes it obvious to anyone looking at the image that its yours.
The easiest way that I have found to do this starts with your image loaded into Photoshop as a native Photoshop PSD file. Lets say that we start with the following image.
First make sure that you have resized the image to the size that you want to display in the browser.
Select New from the Layer menu and create a new layer. It doesn't matter what the layer is called but the opacity of the layer should be set to about 20%.
Next make sure that the layer is selected. Choose a foreground colour that contrasts nicely with the main colour in the centre of the image. Select the text tool and type in your copyright notice.
The image will now look like this.
As you can see, this image does not look noticeably different from the one above but is clearly marked as being Copyright Elaine Bell. A little experimenting with the position of the copyright notice, the colour that you use for it, and even the opacity level of the layer may be necessary in order to obtain the desired results. Depending on the physical size of your screen and its resolution, the notice in this image may or may not be easily readable, an enlarged copy of the same image is shown at the bottom of this page so that you can more clearly see that the message is there. As this copyright notice is actually a part of the image, anyone who steals your image takes the copyright notice along with it.
As an alternative to placing a copyright notice in the image, you might instead decide to place the URL (address) of your web site in the image. This will mean that anyone who steals your image will at least be advertising your site.
For anyone who is interested, the photo used as an example is of Daniel Chapman and 'Archie' taken by Elaine Bell at Thirlemere NSW on Sunday, 10th June 2001.
This article written by Stephen Chapman, Felgall Pty Ltd.