Desktop icon not showing in application menu

On my Kubuntu Laptop, I installed an AppImage and created a .desktop file for it. Now when I start the application the icon is shown in the window bar but not when searching for the application in the application menu. Here are some informations about the icon file I'm using:

PNG image data, 100 x 100, 8-bit/color RGBA, non-interlaced

I don't think I did anything wrong with the icon path, otherwise the icon wouldn't be displayed in the first image.

This is the icon that I'm using

Thanks for your help.