The fact of the matter is, if someone can see an image on their computer monitor, there is no fool-proof way to prevent them from coping the image. However, there are a number of things you can do to make it a big more difficult for them.
By default SiteSpinner and WebEngine disable the Copy tool that appears over large images in IE (to change this setting, go to the Page Editor > Special Effects tab, and check/ucheck the 'No Image Copy Toolbar' function.
Another option is to Watermark your images. To do that, use the Title Editor to create a Title (you can use your name if you like). Skew the Title, place it over your image, adjust the transparency (in the Quick Editor > Outline tab), then select the Title and the Image and create a Permanent Group. Then, if someone copies the image, they will be coping your name along embedded over top of it.
Lastly, you can disable right-clicking of images. This tends to get people angry, and does not prevent someone from stealing the image if they really want to. For more on tis see the discussions here.