Exactly which Mac?
For an all-in-one like a Classic, then no. The best you can do is a telecine where a camera is carefully centered and focused on the display, and has the frequency synchronised to stop rolling-shutter effects.
For any Mac with a monitor output port, then it should be possible to use a hardware capture device, which passes the image through to a monitor. Essentially these "vampire" the image as it goes to the external monitor, saving video or stills to a separate device.
If you're using an AWS mac.metal image, then it can be done to get a still screenshot of them booting, but it's more fiddly and the machine won't wait for you.
Your final option is to search your favourite search engine for an image someone else has already snagged, like this:
From Change the Mac Startup Drive on Boot
by using search terms "mac II boot screen" on Google.