How can I block rightclick or prevent images for download in Muse
Short answer: You can't.
Disabling right click won't prevent people from taking images if they are so inclined. It only annoys people by taking away advanced browser menu options. Anyone can screen grab images with their Print Screen key. Or they can find assets easily by looking at your source code (F12) or going to their Temporary internet folder.
If you really want to protect copyright on images, use digital watermarking.