Sample of how to set an image to be shown in the 3D viewport as a texture

set_image.py

# made for https://stackoverflow.com/q/45586114/2684771
# expects the current screen to have a 3dviewport and image editor visible

import bpy

bpy.context.scene.render.engine = 'BLENDER_RENDER'

for obj in bpy.data.objects:
    bpy.data.objects.remove(obj)

bpy.ops.mesh.primitive_cylinder_add()
bpy.ops.mesh.uv_texture_add()

img = bpy.data.images.new('colgrid', 1024,1024)
img.generated_type = 'COLOR_GRID'

bpy.ops.object.mode_set(mode='EDIT')

for area in bpy.context.screen.areas :
    if area.type == 'IMAGE_EDITOR' :
        area.spaces.active.image = img

bpy.ops.object.mode_set(mode='OBJECT')

for area in bpy.context.screen.areas :
    if area.type == 'VIEW_3D' :
        area.spaces.active.viewport_shade = 'SOLID'
        area.spaces.active.show_textured_solid = True