I have a third-party application, (in this case, Oracle SQLDeveloper which is a java application), that I installed manually (i.e. not through apt). When I ran it, I selected to "Lock to Launcher" so I can easily launch it again. The icon works to launch the application, but the icon is a generic grey question mark icon. I can download various other icons that would be more appropriate, but I don't see any way to customize just a single icon in the launcher.
From your comment, it turns out that there is no icon defined in the
.desktop file. You can use an icon "of your choice", and edit the line:
You will possibly have to log out and back in to make the change appear.
If you found your
.desktop file in
/usr/share/applications, it is good practice to first copy it to
~/.local/share/applications and edit the local copy. After log out / in, the local version will "overrule" the one in
Both Dash and the Unity Launcher will then use the local one.
You might want to look inside the application's directory (which seems to be
/opt/sqldeveloper) if there is not already an appropriate icon to use.
/opt is a typical directory where self-contained "all in one" applications can be located.