Adding Schema.org To Site

I want to add schema.org to my site, I've read some guides for that and I understood the way I should do that. But should I add these tags for example for images and url:

<figure itemprop="associatedMedia" itemscope itemtype="http://schema.org/ImageObject">
  <a itemprop="contentUrl"  target="_blank" rel="nofollow noreferrer" target="_blank" rel="nofollow noreferrer" href="someurl" rel="bookmark">
    <img  src="someurl"/>
  </a>

</figure>

to all my images, all my urls, all my pages or there is a way to do that globally for my site?

Yes, they are meant to be added to all your tags. That way you show search engines the semantic relationship between every item on your webpage.

If you don't use any programming or frameworks, you need to add them by hand.

It is a good practice to always validate them while developing to see how Search Engines will see them: http://www.google.com/webmasters/tools/richsnippets