Is there a difference between
Desktop Backgrounds, Wallpapers
and Desktop Picures?!
The simple answer is, No!
These days it's all the same.
Call it Desktop Backgrounds, Pictures, Images or Wallpapers, Good Wallpaper Sites offer you basically the same thing. It's all JPEG images in different resolutions that fit as your background image on the desktop.
In the very beginning of personal computing, however, there was a major difference between "Desktop Pictures" and "Desktop Wallpapers". They where made to fit different operating systems! This was back in the 80's, eons before the Internet, when Macintosh was calling their backgrounds "Desktop Pictures" and Windows was called their backgrounds "Desktop Wallpapers".
The major difference between the two types of background images in those days was the type of image files the two competitors where using. PCs (Windows) could only display bitmap image files (.bmp) while Macintosh only displayed jpeg (.jpg) image files. The difference between these formats is that jpg's is a compressed image file and takes up much less memory space when it is stored.
Later on Windows realized the benefits with .jpg image files and adapted it as one of their own dominant image file standards. Now, because of Windows dominance on the personal computer market the name "Desktop Wallpapers" for desktop backgrounds became widespread. Once the Internet was born and the first sites offering free backgrounds of .jpg images and called them "free wallpapers" the name became carved in stone.
So most people today simply say "wallpapers" when they talk about desktop backgrounds. A few sites, especially those aimed for offering free backgrounds with Macintosh themes will call their backgrounds "Desktop Pictures". The most neutral name (and the one used in this article) would of course be "Desktop Backgrounds", this is what Wikipedia calls their collection of free images-to-decorate-the-background