Use Marked 2 markdown preprocessor functionality to rewrite each image URL to an absolute path on the system before displaying the preview.

marked-markdown-preprocessor.rb

#!/usr/bin/ruby

require 'pathname'

imgURLRegex = /^(!\[.*?\]\()([^\s\)]*)(\s*.*\))/

rootsPaths = [
	'/Users/username/Path/To/hexo/source/',
	'/Another/PathTo/Website/Directory/'
]
filePath = ENV["MARKED_PATH"]

if RUBY_VERSION.to_f > 1.9
	Encoding.default_external = Encoding::UTF_8
	Encoding.default_internal = Encoding::UTF_8
	content = STDIN.read.force_encoding('utf-8')
else
	content = STDIN.read
end

rootsPaths.each { |root|
	if File.realpath(filePath).start_with?(root)
		# Rewrite every image url with an absolute path on the system
		content.gsub!(imgURLRegex) {|tag|
			tag_parts = Regexp.last_match
			tag_parts[1] + File.expand_path(File.join(root, tag_parts[2])) + tag_parts[3]
			# puts File.join(root, tag_parts[2])
			# puts tag_parts.inspect
		}
		print content
		exit
	end
} 
print 'NOCUSTOM'