Sometimes, PDFs contain images on one or more pages (usually because the program that created the image allowed the author to save the image to PDF).
Though it is better to get the image from the source, that is often not possible. Therefore, if all you have is the PDF version of the image, sometimes your best option is to convert the PDF to an image (or images if there are multiple)!
The PDF viewers typically allow you to Export the PDF page to an image type as shown in this video.
NOTE: for Mac Users
The Preview app allows exporting images:
Bulk Image Export
When I have a lot of images in a lot of PDFs, I use a "PDF to JPG" (from RootRise Technologies -- link)
NOTE on: Adobe Reader DC
As of Nov 2018, at least, you can no longer freely export a PDF image. It seems you have to purchase a subscription to "Export PDF" to unlock options like images... :-(
NOTE: for Windows Users
You can research the latest tools by Googling: ""
This link had some good non-Adobe PDF readers that exported to image. Some were free as well!