but how can i add an image in my header?

This is a setting in your BeeHive theme
Probably under Title Bar

https://themified.com/beehive/documentation/settings.html

Your beehive WP Theme is responsive optimized so it works on all devices.

@media (min-width:576px) {
  .container {
    max-width:540px
  }
}
@media (min-width:768px) {
  .container {
    max-width:720px
  }
}
@media (min-width:992px) {
  .container {
    max-width:960px
  }
}
@media (min-width:1200px) {
  .container {
    max-width:1140px
  }
}

You can change max-width to a higher value if you know what you are doing. 😉 
Insert the customized CSS code
Dashboard > wpForo > Settings > Colors & Styles > Custom CSS

I would like to make forum wider and add an image to my header as per attachment.

Thank you in advance.