I'd like to load a transparent png and have all parts that are transparent show the desktop or underlying windows- is there a Linux image viewer that does this? I'm using Ubuntu and Gnome.
I'm not asking the same as question as this: Linux: image viewer with transparent window?
I was able to do this in Ubuntu with the XFCE desktop using pqiv, which is in the Ubuntu repositories. Here's an example of me displaying a transparent PNG over both my desktop background and Firefox: 
The command I used to generate the transparent window was:
pqiv -c -c -i foo.png
This only worked when I turned "display compositing" on in XFCE. I bet if it works in XFCE there's a way to make it work in GNOME since they have so much in common.
sudo apt-get install qiv
qiv -p (yourimage)
Worked with default window manager under Mate Desktop on Ubuntu 14.04. I used it with an .svg rather than a .png.
The transparency of the window itself is decided upon by the window manager. Which means that maybe it is possible to use compiz to set any image viewer's window to be transparent. This will only partially achieve what you are trying to do, since even the non-transparent parts of the image will be somewhat transparent while the transparent parts will be "more" transparent.
As pointed out in https://superuser.com/a/870183/282396, you can use transset to make any window transparent. Unlike described in that answer I recommend just running transset 0.5 and then clicking on the window you'd like to make transparent. This way you won't need to use any special image view.
