To search for an image based upon a key word, simply use a Google Images search directly:
https://www.google.com/imghp?hl=en [Bookmark it!]
(When you see the initial results, click on 'Tools' and then choose relevant filters, such as 'Size' or 'Type').
To carry out a
reverse search, based upon an image that you've found on the web, right-click on that image and select 'Copy image link'. Go to Google Images (as above), click the camera icon, right-click in the search field, select 'Paste' and click on 'Search by image'.
To carry out a reverse search, based upon an image that's already on your hard drive, click on the camera icon (as above) then on 'Upload an image'. Navigate to the location of your image file and double-click on it.