why can not load gif format file as Image source?

[Simon]

I download the gif from http://www.animatedimages.org/img-animated-button-image-0575-88979.htm, I also attached it here.

my code is simple

from kivy.lang import Builder
from kivy.app import runTouchApp

runTouchApp(Builder.load_string(
'''
Image:
    source:'animated-button-image-0575.gif'
'''
))

the error log is

C:\Python27\python.exe D:/123/effect.py
[INFO   ] [Logger      ] Record log in C:\Users\Administrator\.kivy\logs\kivy_17-09-09_50.txt
[INFO   ] [Kivy        ] v1.10.0
[INFO   ] [Python      ] v2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)]
[INFO   ] [Factory     ] 194 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil, img_gif (img_ffpyplayer ignored)
[INFO   ] [OSC         ] using <thread> for socket
[INFO   ] [Window      ] Provider: sdl2
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] GLEW initialization succeeded
[INFO   ] [GL          ] Backend used <glew>
[INFO   ] [GL          ] OpenGL version <4.0.0 - Build 9.17.10.2932>
[INFO   ] [GL          ] OpenGL vendor <Intel>
[INFO   ] [GL          ] OpenGL renderer <Intel(R) HD Graphics>
[INFO   ] [GL          ] OpenGL parsed version: 4, 0
[INFO   ] [GL          ] Shading version <4.00 - Build 9.17.10.2932>
[INFO   ] [GL          ] Texture max size <8192>
[INFO   ] [GL          ] Texture max units <16>
[INFO   ] [Shader      ] fragment shader: <No errors.>
[INFO   ] [Shader      ] vertex shader: <No errors.>
[INFO   ] [Shader      ] program: <No errors.>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
[ERROR  ] [Image       ] Error loading texture animated-button-image-0575.gif
[INFO   ] [Base        ] Start application main loop
[INFO   ] [Base        ] Leaving application in progress..

I am wondering why error message here, the gif file is ok, at least it seemed ok.
How to solve this problem. thanks in advance.

[Simon]

I solve the problem, though I still do not know why...

my code is

import os
os.environ['KIVY_IMAGE'] = 'sdl2,gif'
import kivy
from kivy.app import runTouchApp
from kivy.lang import Builder

runTouchApp(Builder.load_string(
'''
Image:
    source:'animated-button-image-0575.gif'
'''
))

in fact, I do not know why, I just can not put 'pil' in 
os.environ['KIVY_IMAGE'] = 'sdl2,gif'

and also after this , I have to add

import kivy

to make the config work :)