in jekyll/plugins/markdown_converter.rb [152:160]
def convert_img(el, indent)
attr = el.attr.dup
src = attr['src']
src = @options[:baseurl] + src[1, src.length - 1] if src.start_with?('/') and !src.start_with?('//')
attr['src'] = src
"<img#{html_attributes(attr)} />"
end