Open Google in your browser.  Then on the upper right of the page click on 'Images'.  If you have the image saved to your computer you can use the browse button to find it.  If it is on a web site, like BP or other ad site, you will need to click on the little camera in the search bar, and then copy and paste the URL for the image you want to search.  You can get that URL by opening the image you want to search and right clicking on the image and selecting 'properties'.  The properties dialog will open and you can copy the Address (URL) into the Google search bar.  Hope that helps.
 

using firefox, all you have to do is open both ad and google image, and drag the image onto the search bar.

I dont know if it works the same for chrome or IE