If you're the webmaster:
Delete the image from your website. This would include Facebook, Twitter, Flickr, etc... Once you delete the image, that doesn't mean (as we're sure you've realized) Google has removed the image from their search results. This is because Google has the initial search images stored on their servers and only links to your website when a visitor clicks the "Fullsize image" link or clicks through to your website. This would eventually rectify itself when Google does another crawl of your website, refreshing its data. But you can expedite the process by getting the URL to the image and going to Content Removal and submitting a request for removal (this works for full websites too.)
If you're not the webmaster:
This will prove a bit more difficult as Google doesn't have the right to censor results based on whoever asks them to. Thankfully, we're doubtful many of you are major celebrities who have your paparazzi photos popping up all over Google Image search results so this should be a bit easier. What you'll need to do first is contact the webmaster. Google has provided some helpful hints as to how to go about doing that . Explain to them why you'd like the image taken down and offer a substitute one if they're using your photo for reasonable purpose like a news article. Once they take the image down you will have to repeat the same final step as before by taking the URL of the image and pasting it in that Content Removal page.