Opal uses the rmagick gem for image processing(thumbnail generation, etc.) and special effects(sepia, monochrome, etc.). RMagick is an interface to imagemagick and it's also a native gem, which requires you to have some dependencies installed on your server before it can be installed.
If you get any errors when running bundle install or gem install rmagick, you may need to install these dependencies. Here's how.
If you're using a debian-based linux distribution, such as Ubuntu or Mint, it's easy to install the rmagick dependencies:
sudo apt-get install libmagickwand-dev bundle install rmagick
See this thread for more info.