I have tested this method and it really works great with our Xperia C and it's really simple work. Got this from :
Credit goes to the actual owner and a bit to me for trying and testing on Xperia C
Hope you will enjoy this as we are limited to customizing our phone because of locked Bootloader !!! I am hoping that our phone bootloader gets unlocked soon so that we can have CWM installed and then finally start off with Custom ROMS build.
I have attached 2 boot_logo files which i actually tried
1. Root required with a good explorer like ES Explorer (I use it all the time and i love it)
2. Navigate to System/media/images and you will find boot_logo file. Please take a backup first.
3. Now rename the file to boot_logo.old and copy my attached file there and make sure to give proper permissions rw-r-r using Root Explorer. (also make sure to rename my attached files to just boot_logo)
4. Once done, simply reboot and you will see the changed boot logo image