3D Studio Max. You coul'd do it with any 3D program.. since it have the proper tools. Anyway i presume that everyone here have money enough to burn in ~1000 dola in a promotional offer of 3Dsmax.
. So let start by downloa... ops.. i say: buying your copy of the refered program.
Now you'll need a GI render engine. GI means Global Ilumination, or 'simplifying': Skylight emulation. So i recomend you get a newer version of the MAX (the v9 is the least and is quite bugged, but will do the job) cause it comes with Mental Ray that will make everything.
You'll also need a properly UVmapped model.
And probably lot of patience to understand my consufing english. Any problem with this, just post it HERE, pls don't send me a PM cause your question could be the question of another guy. Ok?
Once you got everything. Start the program and let set the enviroment to your texture lightning get a nice and realistic look.
* Start making a plane to simulate the 'ground'.
* Set Mental Ray as default render engine. Following the pointers.
* Now is time to make the skylight works. Enable the Final Gather
note: if your model has too many concave areas. Probably the lightning will be quite dark. You can solve this by increasing the DIFUSE BOUNCES value to 3. If anyway the texture goes dark, do the same to the MULTIPLIER value. If your problem is quality and not 'darkness', set the PRESET to 'VERY HIGH'
* creat a skylight anywhere on the scene, the position doen't matter. Be sure to assign a WHITE color to the sky. By default the color is a very light blue (like the real sky) but we are making a kind of 'shadow map' so leave everything in gray tones. You could ask me about creat as well a Direct Light simulating sun shadows, but will look unrealistic (like OTA that right side of units are dark and left bright). Let the game engine do this job. Ok? Ok.
* open da material editor pressing 'M' on your keyboard. The browser will open with lots of gray spheres. Simply drag and drop one of those to your model (represented by my box), and aswell to the ground.
* now lets to the magic. Click on your model (UVmapped) and only on it. Now hit '0' (zero) on your keyboard (not the numeric keyboard) the 'Render to Texture' screen will appears. There you can assign a output dir to your texture go and a name and extesion on 'FILE NAME and TYPE.
scroll down and continue... i recomend you render in at least 1024x1024. Will make the GI calculations better, if you will use a 128x128 map, just reduce its size latter on a 2D program. NO the result won't be the same as you rendered with 128x128.
That's all. The file will be on your pre assigned dir. Now you can open your 2D program and use it layers. Hope everything is clear.