In the Alt+Tab list of currently running app icons a hi-res version of an app icon is only displayed if the app has an associated .desktop file (which specifies the icon, say, "icon-name").
The older/general way of setting the icon:
displays a lower-res (blurred) icon in the Alt-Tab menu regardless of the resolution of the loaded icon (in this case a 128x128 icon). Can it be fixed other than with a .desktop file? Is it a Unity/gtk bug or a policy?