1. Pull the downanimation.zip from your phone or from your latest ROM download. It's located in /system/customize/resource, or download downanimation.zip (the download already has the desc.txt file modified)
2. Find your image and resize it to 1080 (width) by 1920 (length), then convert to png.
3. Make 4 copies (5 total) and rename them to power_down_00001.png, power_down_00002.png, and so forth down to power_down_00005.png
4. Depending on how many pictures you are using, modify the desc.txt. (more will be needed to make it actually animated) modify the desc.txt file change then number after the 1920 to roughly half the total number of pics.
5. Zip them back up in the file from step one.
6. On your phone using a root explorer navigate to /system/customize/resource and rename the original downanimation.zip (just in case you want to easily go back to it)
7. Copy the new downanimation.zip to your phone then using root explorer copy it to /system/customize/resource and make sure the permissions are like the original.
(alternate step 7) (pointed out by stillthisguy in post #4) If you have ViperDNA by nit then you can go into Venom Tweaks>Advanced>Custom downanimation. It will find all of your .zip files and you just choose the one you want to use.
The first request is done and attached. (be sure and rename to downanimation.zip)
Note: I'm sure there are better ways to do most of this, so as i get suggestions then i'll try to modify the instructions to the best.