There are many good reasons why you should use the specialised image databases and multi-media collections such as Artstor or BoB Box of Broadcasts in preference to using Google:
Google does not contain all images, for example, many organisations only make their images available through licensed collections or specialised image databases such as Bridgeman or Artstor.
Accuracy and Searching Power
Specialist image databases often have more sophisticated search methods, such as by creator, medium, technique, location, colour etc.
It is not easy to tell which images have been modified. It is very easy to alter images online in all sorts of ways. If you need to be certain you are looking at a faithful digital version of an image, use a specialised image database such as Bridgeman or Artstor.
Just because someone has uploaded an image to Google does not mean you have permission to copy and use it. It is not easy to tell which images on Google are in copyright and which are in the public domain or covered by creative commons licences. If you use a specialised image databases such as Bridgeman or Artstor you can be confident that you can use the image in your essays or presentations.