I did the image dump of an X window by using
xwd -root -silent > screen.xwd
Now, how do I open this file or convert into JPEG file?
Probably I could use
pnmtojpeg, but it seems it is not available in Homebrew.
Alternatively use ImageMagick which support jpeg, png and other popular formats, example usage:
import -window root screen.jpeg
brew install imagemagick --with-x11