You would need an application that converts video to jpeg , Resolution depends on your phone i dont remember the resolution of the S3 off the top of my head.
while using the application to convert the images you need to have them named consecutively so its known in what order they will be played something like 0000.jpg....00001.jpg .....00002.jpg......00003.jpg.....etc i dont know what the limit is but i have made one up to 200 jpgs. these pictures need to go into a folder called part0.,,, then you can have a second part "part1" with the ending of the video
you will also need a file called desc.txt, in this txt file you will name the resolution frame rate, how many times the part will loop, how long to pause inbetween switching to next part, which part will play then repeat.
Here is what the desc.txt file should look like, i will attach a boot animation to download
HERE as an example actually you can replace the contents adjust the desc.txt for your phone and zip it.then place the bootanimation zip to "/system/media/" location on your phone
[NOTE]This will not work on a Stock Rom. you most likely will need a kernel that supports custom boot animation. find a custom kernel you like and ask the developer if it supports custom boot animation.[/NOTE]