How do I print images onto minifig body parts?
You can do that with scale model water decals. You can buy that in blank sheets and print out your designs with an inkjet printer. You can also use products that help those water decals set (adhere) and conform to the surface deformation and use scale model varnishes to seal it in place.  

Shopping list:


Water decal sheets
Setting solution (sticks better)
Solvent solution (conforms to the surface)
Varnish/Finish


As stated in this other question about painting Lego bricks: Don't expect it to sustain the normal usage of LEGO bricks. This is good for display or light usage (like playing D&amp;D with custom minifigures)