I am new to Unity. Is it possible to just use a jpeg or a picture format as your game model; i.e. Jet fighter or soldier as a model? Or, is this in possible and needs to be made out of polygon's.
You can create a material with a transparent shader, and then assign your picture as the main texture, then apply your material to a plane and you are ready to go. Set the camera to orthographic if you want to make it completely 2D. It could also be rendered using the GUI or the sprite manager on the wiki (I think it is on the wiki anyway, otherwise try to search on the forum).
You want to build a 2D game? Lots of people seem to be positive of SpriteManager 2, that can be bought for a reasonable price, SpriteManager 1 is free.
If you want to do it yourself it's useful to have one model, a simple plane (you can use the buildin plane as well but it has more vertices than necessary), Place the plane in scene, add a face to camera script to it, create a new Material with transparent shader for any texture you want to use and set the texture to the material, choose the right material for the plane in the scene (but this is basically what previous poster already stated)!