There is no difference between the .jpg and .jpeg filename extensions. The two terms have the same meaning and are interchangeable. The reason for the different file extensions dates back to the early versions of Windows. The original file extension was .jpeg; however in Windows all files required a three letter file extension. So, the file extension was shortened to .jpg. Now, the most commonly accepted and used form is the ‘.jpg’, as many users were Windows users. Imaging applications, such as Adobe Photoshop, save all JPEG files with a ".jpg" extension on both Mac and Windows, in an attempt to avoid confusion.
Generally speaking, JPEG should be preferred because it offers the best compromise between the quality and the size of the images. Particularly, if you want your website to display quickly, you should choose JPEG because it offers small image sizes, which are less heavy to load. If you put more emphasis on the quality of the images, PNEG will be then the best choice.