When running under Gnome Picard the Alt-Tab application switcher does not show an application name. It also does not group multiple Picard windows as it should. See picard-gnome-alttab.png. This is caused by Gnome not being able to match the running window instances to Picard and its .desktop file. Another effect of this is, that once Picard is running you can neither pin it to the dock nor open a new window by right clicking on the dock item.
I have a patch ready that will fix this behavior.
I don't know about other desktop environments, but they could have similar issues and this fix could also helpt in those cases.