Without specific tools from each motherboard manufacturer (or at least every motherboard BIOS chipset manufacturer) then there is no program that can do this in a generic fashion.
Each BIOS firmware will have information on display setup and, as you have seen, sometimes for particularly large manufacturers an image.
Somewhere in that firmware image you download from a motherboard manufacturer will be some data corresponding to that image that is potentially editable by humans but it may well be in a "raw" graphics format that no sensible "desktop" Paint like image editor will touch.
Once you have that image, and have edited it you then have the problem of putting it back into the firmware image for flashing back onto the motherboard. In that firmware image is most likely to be a CRC checksum of some kind to make sure that the data is in tact and the BIOS firmware is not broken. You'll then need to recalculate the CRC and stick that in the firmware file too.
Then you might be able to flash that BIOS to the motherboard you have and it might not kill your motherboard, if you're lucky.
Now another motherboard of a different type comes through the door. You can't just flash your modified firmware to this new board because the BIOS chip is different due to hardware changes. You'll keep meeting this problem with almost every motherboard you see, if you want them all to look the same then you either have to be the one dictating the BIOS software to begin with (like Dell can) or you face a world of pain hacking and kludging your way through them all. Each with their slightly different formats for the code and image data, all stored in different locations within the firmware image.
Some people (like Asus) are kind and give you the tools, though most companies think it's a bit risky giving you those tools because it is more likely that you'll be regularly flashing your BIOS into an early grave and thus complain at them for a replacement because you were changing your boot screen every day.
Anyway, why should any normal user care about the BIOS boot screen? No real user cares that much, the screen is only there for 10 seconds at most and provides almost no real information that a non-technical person will find useful or interesting.
You either need to be the one in power or the one in pain, you are not Dell.
If you desperately have to have the image in the BIOS screen the same across a lot of PCs you are best off standardising to one type of motherboard and then going to the manufacturer of that motherboard and asking them to customise it for you. Chances are this will cost you though.
