The Last Supper, painted by Leonardo da Vinci is the highest resolution picture. It features a resolution of 172181 X 93611 (16,118,035,591 pixels). The picture was made by putting together 1677 separate images. This allows you to view the picture like you were standing right next to it.

You can view the picture here

http://www.haltadefinizione.com/en/cenacolo/look.asp

The picture below was taken by the Hubble Telescope. This picture has a resolution of 22620 X 15200 (343,824,000 pixels) and is 706MB in size.

The original picture (22620 X 15200, 706MB) can be viewed here

http://www.spacetelescope.org/images/original/heic0710a.tif

A reduced version (4000 X 2688, 6.2MB) can be viewed here

http://www.spacetelescope.org/images/publicationjpg/heic0710a.jpg