Icons are used within an MPS editor. The relative path information to the required icon file differs depending upon whether you are working within the MPS development environment or in the standalone application. Consequently, the icons are either only visible in the standalone application or only during development. Of course, neither is a satisfactory solution. 

Troubleshooting: 

To get this situation under control, it is advisable not to reference the icon files used via the file system but from a Java resource. 

To do this, simply create corresponding "Dummy Actions" within a suitable Solution, within which the required icons are referenced. The build environment then ensures that the icons can be found within the respective generated JAR files.