The 8-bit framebuffer images can be read directly by some paint programs if you tell it the width and height. On a K4, or a K5 booted from diags, it is 8-bit grayscale 600x800. On a K5 booted from main it is 608x800. It would be possible to create a simple text-only PGM header for each format, and append the framebuffer copy to that so paint programs do not need to be told the image resolution.
The K3 is a little more complex because it has 2 pixels per byte. You can get a general idea of what the image looks like by loading it into a paint program as a RAW 300x800 image. But better, dpavlin has a convertor to that reads framebuffer images and writes a standard image format. You can find it in the tools index or K3 index I think...
You can do all of these things from a script that can be installed in the GUI launcher menu, or from launchpad, or from the kite launcher.