Yep, that little dot in the sea of infinity is our little blue marble. No one have ever been more than a pixel (EDIT: a couple hundred pixels) distant from it, and soon even that achievement would fade from living memory.
Farthest human has been is 400,000 km [2]. So the farthest we have ever been is around 160 pixels.
[1] https://www.google.co.in/search?q=diameter+of+earth&oq=diame...
[2] https://en.wikipedia.org/wiki/List_of_spaceflight_records#Fa...
And note they describe that small picture as "zoomed in", So I figure 400km is about 23 or maybe 24 pixels at most =D (or a diameter of less than 50 pixels).
Waiting for someone to find an even better estimation!
You cannot use the earth-to-moon distance, unless you also know the exact position of the camera, earth and moon in the 3D plane.