Finally, you can now have image search results as part of the Google AJAX Search API. Take a look at the documentation or this page's source code to get started; to reproduce, all you need is your own API key. (Unfortunately, "setSiteRestriction" doesn't seem to work with images, so I'm adding the site operator. Another problem is that the thumbnails are downsized on the client-side, meaning that they're not only smaller than the Google Image search thumbnails, but also using potentially sub-optimal resizing; you may be able to override this with your own CSS though.)
If you want to provide an image search but do the coding on the server-side, you can also go for the Yahoo Image Search API (note however that Yahoo's image adult filter doesn't work well).