How to set background image?

Your CSS code is almost correct, just there is an incorrect selector (.your-form). In every form created by Forminix are displayed inside a class (.forminix_single_form). So you can modify the CSS this way:

.forminix_single_form {
  background: url(https://img.freepik.com/free-photo/grunge-paint-background_1409-1337.jpg);
  background-repeat: no-repeat;
  background-size: cover;
}

Hope this will let you achieve what you wanted.