I have a background Image in my Main Menu but when I add a Particle System it plays behind the image.
This is the first time I used Particle and Im a beginner. Please help 
Use a material with a special shader. That shader (the algorithm, telling material how to shade pixels that object represents) must be rendering particles the normal way, except in Tags{} it should have "Queue" = "Overlay"
Set the sorting layer of the particle’s renderer to be a higher layer than the image’s renderer. You can define sorting layers in Edit → Project Settings → Tags and Layers.