Google Image Search has a new option that allows you to restrict the results to photos: just select "Return images that contain photo content" from the advanced search page.
A simple way to find photos would be to restrict the results to JPEG files, but a search for Gmail shows that many people use the JPEG format for logos and screenshots. If we use Google's image analysis technology, we'll find more photos related to Gmail, including the Gmail soap, Gmail Theater and Gmail's product manager Keith Coleman.
The other two content restrictions available in Google Image Search are for images that contain faces and for images that illustrate recent news articles. Microsoft's image search engine has more advanced options for refining your search: you can find photos, illustrations, faces and portraits.