That is a good question.
If you are at the BIOS, you do not yet have an OS, you have a very rudimentary (think amoeba) file system, and memory management is at an unkown stage.
In other words, the PC that is displaying the BIOS is not going to make the screenshot, period.
However, a digital camera will, and if you take a look at the various harware sites out there (tom's harware, anandtech, ars technica, etc), you will see that that is their solution as well.