I can confirm you have found what you were searching for (checking the
panel's properties of my system).
I think I found what I was looking for:
/usr/share/themes/Trisquel/gtk-2.0/Panel/panel-bg.png
In case anyone else loses it :)
Hello, what is the image of the main Gnome panel by default? I removed my
panels and now, trying to get them back, that image is no longer there.
If you don't know what I'm talking about, you can find out by right-clicking
the panel, selecting Properties, clicking the Background tab and seein