Another method worth mentioning is the use of depthmaps.
For 10x15cm fotoprints five or six depth levels combined with a depth
gradient works well for most of my convertions.
Initialy I used GLtip to create the depthgradients but after some
experimenting I learned to do it in my graphic editor (PL32).
I use the free Bas-relief program for the actual 2D3D convertion
http://www.3dmix.
Another nice tool is the Recolored program
http://www.recolore
With some luck you can find the last free version.
It realy is a gem !