This code use the Blender Python API to batch render 3D model to image file. It is only tested on Blender 2.64.

obj2png.py

import os
import bpy

# put the location to the folder where the objs are located here in this fashion
path_to_obj_dir = os.path.join('D:\\', 'Workspace\\Data\\scapecomp')

# get list of all files in directory
file_list = sorted(os.listdir(path_to_obj_dir))

# get a list of files ending in 'obj'
obj_list = [item for item in file_list if item[-3:] == 'obj']

# loop through the strings in obj_list and add the files to the scene
for item in obj_list:
    # full path and file name
    path_to_file = os.path.join(path_to_obj_dir, item)
    # import ply into the scene
    bpy.ops.import_scene.obj(filepath = path_to_file)
    # change suffix to ".png"
    portion = os.path.splitext(item)
    newname = portion[0]+".png"
    # apply current material to this object
    bpy.ops.object.material_slot_copy()
    # shade mode
    bpy.ops.object.shade_flat()
    #bpy.ops.object.shade_smooth()
    # translate and rotate
    #bpy.ops.transform.rotate(value=(30,), axis=(0.0, 1.0, 0.0), constraint_axis=(False, False, False))
    #bpy.ops.transform.resize(value=(5, 5, 5), constraint_axis=(False, False, False))
    # new full path and filename
    path_to_file2 = os.path.join(path_to_obj_dir, newname)
    # set output render filepath
    bpy.data.scenes['Scene'].render.filepath = path_to_file2
    bpy.ops.render.render( write_still=True )
    #bpy.ops.export_scene.obj(filepath = path_to_file2)
    # delete current object from the scene
    bpy.ops.object.delete()

how we can adjust the camera for every object file and every object file have a different size so how's is it possible?