How to render 2d sprites in a 3d world?

In opengl how do I render 2d sprites in opengl given that I have a png of the sprite? See images as an example of the effect I’d like to achieve. Also I would like to overlay weapons on the screen like the rifle in the bottom image. Does anyone know how I would achieve the two effects? Any help is greatly achieved.


I hope this link can help you :

http://basic4gl.wikispaces.com/2D+Drawing+in+OpenGL