Sure, if you have all the needed tools to clear the ROM chip and to re-flash it, and you actually know enough about the hardware to do all the additional code. In short: no one is stupid enough to start doing such.

First of all, there ROM is usually only a few hundred kilobytes big and if there is any space left after the actual ROM image there's only likely about a few tens of kilobytes left at max. Sure, you could perhaps store ONE jpeg-file there, but that's it.

Secondly, the flash chip where the ROM resides is not the same kind of chip that's used in f.ex. USB flash sticks. The ROM chip is not meant to be reprogrammed, and it definitely is not suitable for random read/write access. If the chip can be re-flashed at all the whole chip usually needs to be flashed at once, not just some parts of it.

I ain't laughing since it's clear you just don't know anything about these things, but let's just say what you have in mind is in no way feasible and you should just forget it.