[Galaxy Note5 Q&A, Help & Troubleshooting] zygh0st:
Actually, there isn't anything in the system partition that pertains to the initial graphic shown when the phone starts, that's actually part of the partition known as param, which is little more than a tar archive that's been written to the block device that is designated as PARAM directly. You can use dd to dump it and untar the resulting file with, you guessed it, tar. The images within the archive are png format, but you have to be careful to pay close attention to the very specific palette and compression used in the original images because if you fail to match it exactly then you run the very real risk of rendering the device bricked in an unrecoverable way as the only way to write the PARAM partition is via an environment where you can access dd, or any other utility that is capable of performing direct read and write to block devices, which is either a custom recovery or the running system itself and since it holds the logo shown immediately upon starting the device before the kernel itself or recovery then making a mistake is not an option. Said archive also holds the graphics for download mode as well as the battery image shown prior to the charging animation, which is loaded by the kernel and is a low power mode that partially boots the device and stops just before zygote is brought up, which is the base of the GUI if you were unaware. I've successfully modified every single image in the archive itself and I'd be glad to upload proof to that end as soon as I can borrow another device to capture the result with as I've only got my phone on hand at the moment. I can provide you with more detailed steps, specifically the arguments I used with the convert command from ImageMagick to ensure the image palette and compression were correct since I couldn't get GIMP to save the edited images with the necessary options.

As far as the qmg files, there are several tutorials on the forums that do a great job of detailing that process, but it requires access to a Windows machine as Samsung doesn't see fit to develop a cross-platform tool. Having said that, this is all under the assumption that you're familiar with Linux and a couple of common utilities within it, I don't use Windows save for audio production and the occasional Octoplus unlock, so I can't offer quite as precise information on performing the steps I've mentioned from that environment. I'll go ahead and dig up the arguments I mentioned for the convert utility and post them since I don't recall them from memory as I did this a few months ago. Suffice to say, I know what I'm doing and what I'm talking about and the first order of your request is most definitely possible though I've yet to have ever encountered any instruction on it or even mention of it until now aside from my own venture into that scary process.

It's worth noting that your idea of deterring theft is sound only if the thief would try to sell it and only if the buyer was a noble person, and most thieves don't keep company with good people so it would be something they got a chuckle out of at worst and a minor annoyance at best. Consider how many items must be stolen that have been engraved or otherwise marked and are sold without much ado over it, if they want to steal it they will and not think twice over it. Just my two cents there.