Answer:
PDF is a "printable document format". It can contain pictures but it is not a picture. JPG is a picture format.
You can convert in several ways but you should really look up and understand the difference between the two formats and their appropriate uses otherwise your conversion might not do what you want it to.
Method 1: Print Screen button on your keyboard
This is simply easy and all you need to do is copy the printed image into Windows Paint and save it as a JPG file.
Method 2: Get the whole image of a PDF page - PDF to JPG Converter
However as you are getting the whole image of the PDF page, most of times it is impossible to get the image clear enough when using the "PrtSc" button.
If you would like to get a high resolution JPG image of a PDF page and upload it into your web album or do other things with a clear enough JPG, you need to find a PDF to JPG Converter now. Below I will offer you one that can deal with encrypted PDF files as well.
Method 3: Get a certain image from the PDF
In this case, you can open the PDF in Adobe reader, right click on the specific image, choose "Copy image" and save it into Windows Paint as a JPG.
Method 4: Extract image from the PDF
You may also find some of the images cannot be copied in a PDF, at this time, search online for a free image extractor like "Snagit" and get the image which you want.