Further to Jno's post, if the picture is already on the web, right-click on it and select 'Copy Image Location'. (That's how the option is described in Firefox. It might be slightly different in other browsers). Go to Google Images (as per Jno's link), click on the camera symbol, right-click in the box, select 'Paste'and click 'Search by image'.
As an example, I used that method to answer a question here:
http://www.theanswerbank.co.uk/Competitions/Question1538263.html
If the picture is on your hard drive, open the folder which it's in and drag the image into the box on the Google Images website. (You'll need to make your browser window smaller, so that you can see your browser's window and the folder's window side-by-side). Alternatively you can click 'Upload an image here' and then browse your hard drive's contents as normal.
Note that Google Images seeks other examples of exactly the same image (or of very similar images) and can also offer a 'best guess' at what is shown in the picture but here's no guarantee that it will get it right. Your own avatar is simply identified as "scratching head confused" but looking at the links under 'Pages that include matching images' would be enough to identify the head-scratcher in question ;-)
http://tinyurl.com/j4qpsop