Ubuntu – how to use an image for the custom .desktop file for unity previews

I just ran into the same problem that you did. I messed around with a bunch of settings within my .desktop files to try to get the name to show up in the Unity launcher, since I had created the .desktop files in ~/Desktop then moved them to ~/.local/share/applications

All I had to do to get the Unity application launcher to show the names is log out of Ubuntu and log back in. Pretty lame, I know, but it worked.

At least this shows that in my case (and probably yours too) the .desktop files were not somehow wrong. After all, in my case, the desktop-file-validate program didn't list any errors. There was probably some name cache within Unity that needed to be cleared. It would be more satisfying to know how to do that without logging out, but this workaround is good enough for me for now. Hope this observation helps someone else!

None of the answers provided up to this point have included information about how you can do this in a non-interactive manner. What if I want to script the creation of a launcher and make it trusted without having to click on it?

This usage case is already addressed in this question with this answer. I'm duplicating this information here because this question was much easier to find, and it is not strictly a duplicate because the question here is larger in scope.

To set a launcher as trusted from the command line, run:

gio set /path/to/your/launcher.desktop "metadata::trusted" yes

With that done, press F5 on the desktop or in the current folder to refresh the view. The launcher should now be trusted.

There are certain conditions that must be met (ownership, running programs, running gio as the correct user). See Sander's answer for the full list.