I think Google image search is great. It helps you find out where a particular image comes from and sometimes images related to it. Some forums use attachments and you can't necessarily see if you aren't logged in, and that Google can't access. You have save this image and then upload it to Google. Sometimes you also have images in power point or word documents that you wonder where they came from. Most every OS has some sort of print screen functionality to easily capture pictures from any application or even from a movie. In Windows its print screen or alt+print screen if you want to capture a single window.
I wrote a small web app that allow you to paste an image and crop it so you can search for that part of an image. It stores the image in memcache so you don't need to worry about the image being stored well except by Google.
It only works on chrome and Firefox since AFAIK the other browsers seem to have issues with copy and pasting of images.
http://gimagesearch.com
