What you are asking for here is not physically possible.1 If your target BIOS does not have a built in way to capture screen-shots, there is no way for an external piece of software to do so.
- The BIOS in most PCs runs below everything else on the system.
- Although the data registers can sometimes be accessed, it is not possible to access the BIOS application interface from a higher level in the system (such as from inside an Operating System)
- The BIOS itself is incapable of running external applications.
What you expect to have happen simply won't work. You need to look into alternative solutions. There are at least three ways you could approach this:
A BIOS Emulator
Not to be confused with a Virtual Machine! Most VM systems have BIOS'es of their own that interface between the guest operating system and the host virtualization software, but this is not the same thing as your computers BIOS. Any screen-shots you take of that will be of the VM software's pretend BIOS, not the real one on your system. They are almost certainly quite different.
What you need is a specialty piece of software for emulating the specific BIOS of the machines you are targeting. These exist for many BIOS manufactures and can often read the BIOS flash update files, open them up and run them it a test environment. Basically this is a Virtual Machine host specifically for BIOSes. However, these are very specific to the make and model of your BIOS manufacture. As these vary widely you will need to look into the exact make of motherboard you need to give out instructions for and find and emulator for than BIOS.
Video Output Capture
This is a hardware rather than software solution, but one way to accomplish this would be with a video capture device that you plug in in place of a monitor. For example a VGA2USB capture device or a network enabled remote KVM controller would allow you to read and record the actual picture being sent out on the video output (to what the BIOS assumes is a monitor).
Use somebody else's screen-shots. Google image search is your friend here, as are PDF manuals from motherboard manufactures. You can usually find shots of any screen in almost any BIOS system floating around out there already. They won't have your settings, but they will show the screen layout and you can either Photoshop them to read the way you want them to or simply annotate them with an overlay pointing out where changes should be made.
What solution you settle on will depend in large part how much time and effort you want to sink into this and how important it is to capture the exact configuration as used on your target systems.
One final word of warning: Do not rely entirely on screenshots for this! Of course the screenshots will make it a lot easier for most people to follow along, but if you fail to also write good copy that describes the steps that must be taken, the end result will be brittle and self-obsoleting. BIOS updates will come along with slightly different layouts and obsolete your screen-shots. People will end up with different motherboards than the ones you targeted and won't know what setting they are even looking for. Unicorns will be lost in space with no oxygen masks.2 Screenshots should be a supplement to not a replacement for real instructional copy.
1 At least for most legacy BIOSes; newer UEFI systems make this technologically possible but I don't know of any implementations yet.
2 Just checking you are actually reading rather than just skimming. This warning is important, don't skip it.