this doesnt make much sens to me.
a 2d image, ok no problem.
on top a 3d object, ok no problem but what exactly do you expect the camera to do?
on it a 2d image, on what, the 3d object? as texture? or just a flat image above the 3d object ?

tell processing images dont need a cam? What do you mean by that ?

ok sorry ill clear it up

put it in background.
you can not only define a color you can also call an image like this

background(img);
make sure that it has the same size as your sketch though.

but what if i have anoder image on top of it all?

You can reset the view to the default camera by calling: