Better: create in dom0 a file containing:
qvm-copy-to-vm \(zenity \-\-entry \-\-title='Send to VM' \-\-text='Destination
Save that as an executable script, such as "~/.local/bin/send-to-vm.sh".
Then, open dom0 file manager, right click any png, click open with other
application, and under "use a custom command" enter "send-to-vm.sh %s".
This "registers" the script in the application list.
Then, when you take a screenshot, instead of choosing "save", choose
"open with..." and see if your script shows up in the list of available
applications. If still not, you might have to write a simple .desktop
file in ~/.local/share/applications in order for it to show
up as an option.