A picture you put into a graphic piece (mostly sigs). Say, a real person, an anime pic or some sort of vehicle, etc.
A render (the real term) is the first compile of a model in games. The compiling is called "rendering" if I'm correct. In photoshop however, rendering is cutting out a model out of a screenshot or something else, giving it a transparant background, making it possible to just copy paste it over something else so you can make it seem that model is really there.
Its an image where the background has been cut out so you can see your background behind you, we have some really talented renderers here on GR.
A render is an image that contains just the character/object and not the background. I think that's the simplest and easiest to understand definition.
a render in graphics language is many things. Some call picture cut-outs renders, i however, call them stocks. I see renders as models created in programs like C4d, Bryce, and similar programs.