fbdump is a simple tool that captures the contents of the visible portion of
the Linux framebuffer device and writes it to the standard output as a PPM
file. In other words, it takes a screenshot of anything running on the
framebuffer. It currently has fairly complete support for packed-pixel
framebuffer types and also works with the VGA16 framebuffer driver.

To get a popular image format, issue a command like
'fbdump | pnmtopng > shot.png'