Well, let's cut to the chase. Following method was inspired by this guide.
Credit goes to @tys0n for original guide.
Programs/scripts needed for this method:
Choose graphical editor by yourself as editiing picture won't be described here.
1) Create image image_new.jpg with resolution of 480x800 pixels and size not bigger than 64673 bytes.
2) Take image that you want to replace from /mnt/.lfs (logo.jpg for example, which is bootlogo) and rename it to image_orig.jpg .
Although using this method any picture in param.lfs can be replaced (but with not bigger size than original ones), logo.jpg is only was tested by author.
3) Then dd your param.lfs:
dd if=/dev/block/mmcblk0p1 of=/sdcard/param.lfs bs=512000
5) run param_editor.py. If everything was done successfully, in the same folder param_mod.lfs will appear, otherwise message that describe why it has failed will be shown.
Okay, lets to test param_mod.lfs. Of course, you can flash and reboot once it done, but there's a simple method(without having to wait until bootlogo appears... or doesn't) to check whether param.lfs is correct:
1) unmount /mnt/.lfs:
Will be careful with typing following command. DON'T type wrong number of partition, otherwise your phone probably will be bricked. Better if you just copy/paste this line.
dd if=/sdcard/param_mod.lfs of=/dev/block/mmcblk0p1 bs=512000
mount -t j4fs /dev/block/mmcblk0p1 /mnt/.lfs
5) how to restore original param.lfs? Flash param.lfs, dd'd in step 3 ("3) Then dd your param.lfs:")
ToDo: despite of that I have successfully replaced bootlogo, one thing I wanted to check is how to repair the phone in case if something went wrong. For some reason I was not able to flash neither param.tar.md5 nor param.tar . I had to flash three-file stock 4.1.2 to restore it. I would be glad if anyone will provide flashable param.tar.md5 to avoid flashing stock for those who have failed to change bootlogo.
Okay, lets see what bootlogo can be done(custom param.lfs attached below):
Why exactly this bull***t (or whatever you can think)? Answer is that everything that I've done (including this guide) is just for fun, and it's first idea that came to my mind. Have fun