Hey, I've had this as my desktop background for almost a decade now! Easily my favorite picture of anything "space".

As for your question, it looks like Pluto has an atmosphere in the picture because, well, it does. Although it's very thin, Pluto's atmosphere is primarily nitrogen, so if you stood in the surface during daytime the sky would appear a very familiar blue color.

Here's another shot taken by New Horizons where you can see it much more clearly: https://upload.wikimedia.org/wikipedia/commons/0/02/Blue_hazes_over_backlit_Pluto.jpg

That's a friggin planet....

Short answer? Because it does. It doesn't take much to be visible compared to space.

(Old)

Hey, fuck you this only happened ... checks watch ... 9 years ago? God Damn, we are getting old.

The Old Adventures of the New Horizons

Probably reflection of light from the other side if I had to guess.