(Little) problem with .desktop configuration file

I just installed mGBA 0.6.3 downloading .deb files and running dpkg -i. After that, I tried to create a .desktop file for it, and it seems to work: in Dash there is the correct icon and executable runs without issues.

Then I've dropped it on Desktop as I did with other apps in Dash but it shows a default icon and not the one I chose. What should I do to fix this little problem? I'm currently running Ubuntu 16.04.5; ask me for any other piece of information needed.

In this case I do not think it is a duplicate of "Icon missing when application is launched", because the icon is correctly displayed in the Launcher, in Dash but not as icon when dropped to Desktop (xprop output, in fact, is 'not found').

.desktop file full text:

[Desktop Entry]
Version=1.0
Name=mGBA
Exec=mgba-qt
Icon=/usr/share/icons/hicolor/512x512/apps/mgba.png
Type=Application
Terminal=false
Categories=Game;