Das Werkstatt

UPDATE:
This is obsolete!
There's a way better way to do it, which seems to exist out-of-the-box in Xubuntu 16.04.
See details in replied thread below.

Here's a slightly adapted/improved version of this answer on AskUbuntu. about how to mount disk images (ISO) from the GUI file explorer Thunar in XFCE/Xubuntu.

In Thunar, add a new Custom Action:

Edit > Configure custom actions > (+)PLUS icon

Then, enter the following to execute as command: This creates a folder per ISO file in /tmp, then mounts it (fuseiso) and creates a symbolic link on the Desktop.

Beware: You have to unmount the images manually after you're done.

When you right-click an ISO image file, to select "Open With > Open With Other Application", choose the option "Disk Image Mounter": Now, the mounted ISO will just appear like any other external storage media (e.g. usb stick) in the "Devices" panel, left in Thunar.
For unmounting, simply right-click and unmount the image name there


Thanks XFCE-devs!