I have just added a second (larger) monitor to my Ubuntu box, when I take a multi-screen screenshot with (eg)
xfce4-screenshooter I get garbage in the corner which is normally not seen:
Note the scrambled, garbage from a previously viewed webpage.
Desktop uses Openbox, Nitrogen and LXpanel. This isn't (or shouldn't be, I haven't tested Debian) specific to Ubuntu.
How can I take a screenshot without corner garbage?