There are two methods that come to mind. You could use 'billboards' or you could make a 3D mesh that has the image on it. In case you don't already know I will explain it in more detail.
Billboards are flat planes that you design to always face the camera. This method is cheap to use (performance wise), but in most 3D games, they don't look very convincing.
The 3D mesh I use in my 3D games (not that I have many) is cris-crossing planes that have the same image on each of the planes. I don't rotate the mesh to face the camera but the actual depth of the mesh gives it a more realistic result when compared to billboards.
The fastest code is the code never written.