[Darth Caedus]

I have had issues with trying to over-write a few of the rom files in the windows directory. However, I've found that if you rename the file before copying it and then change it back after putting it in the windows directory it works. So in your case it would be:
1) change the name of your new file to Ani_Poweroff1.gif
2) copy Ani_Poweroff1.gif into the windows directory
3) change the name of Ani_Poweroff1.gif back to Ani_Poweroff.gif
4) wait a few seconds to make sure the changes stick (i usually give it 30 seconds) and soft reset the phone and you should be good to go