Having trouble creating my custom boot animation???

Hey I'm new to this forum but not to customizing my Droid. I've installed custom roms and custom boot animations before but i wanted to try to make my own and every guide I have read all say the same thing, but when I plug it in to my system/media folder in the root of my phone, I reboot and just get a blank screen for 5 sec, then my phones lock screen appears. Anyone have any ideas whats going wrong? Anyone who has created a custom boot animation I'd love for your input.....