Concerning the problem of empty icon, i suppose it is caused by the fact that quick-launch needs to resize the image to fit in the status bar...
So verify that the desktop file refers to an icon in
/usr/share/icons/hicolor/scalable/apps
or to an icon available in multiple sizes (in the right folders).

I was trying to use a customized app.desktop file with a specific icon saved in /opt/app/file.png and it wasn't working until i copied the same file in the previous (/usr/share/icons/hicolor/scalable/apps) path.