Cannot open file ruby.png in Gosu using Learn Game Programming with Ruby

I'm using the Learn Game Programming with Ruby book and I'm trying to just execute the sample code.

I get the following error, using the sample code.

❯ ruby WhackARuby/WhackARuby_1/whack_a_ruby.rb                                                                                                           code
/Users/noahclark/.rvm/gems/ruby-2.2.1/gems/gosu-0.10.4/lib/gosu/patches.rb:40:in `initialize': Cannot open file ruby.png (RuntimeError)
        from /Users/noahclark/.rvm/gems/ruby-2.2.1/gems/gosuu0.10.4/lib/gosu/patches.rb:40:in `initialize'
        from WhackARuby/WhackARuby_1/whack_a_ruby.rb:15:in `new'
        from WhackARuby/WhackARuby_1/whack_a_ruby.rb:15:in `initialize'
        from WhackARuby/WhackARuby_1/whack_a_ruby.rb:70:in `new'
        from WhackARuby/WhackARuby_1/whack_a_ruby.rb:70:in `<main>'

The sample code looks like this:

require 'gosu'

class WhackARuby < Gosu::Window
  def initialize
    super(800, 600)
    self.caption = 'Whack the Ruby!'
    @image = Gosu::Image.new('ruby.png')
  end
end

Any thoughts on what could be going on here? I've tried changing the offending line to @image = Gosu::Image.new('./ruby.png') for example and that didn't help.

I doubt this is the cause, but my ruby version is ruby 2.2.1p85 (2015-02-26 revision 49769) [x86_64-darwin14]

EDIT includes file path:

Invariably the problem is because the file doesn't exist where you think it is.

There are many ways to reference a file. The File documentation has expand_path, realpath, absolute_path, all of which make it easy to reference a file based on an absolute or relative path, and relative to the currently running file, application or a particular directory. How to use them is covered in their examples.

It's important to make sure you know what directory the code considers it's current-working-directory, and/or where the file is. The first is important when using a relative path, and the second is if you don't want to care about your current path and know that the file ALWAYS exists in a certain place.

And, then there's also the case when the file's name is different than what you think it is, or it doesn't even exist.