Google Diagram

Google Diagram or Drawing:

In Google Docs you can create a drawing simply. "Create New" > "Drawing"

As a Drawing or Diagram cannot be Inserted from Google Sites directly. 

You must embed the HTML code created when you publish the drawing to the Web "Share" Tab > "Publish to the Web"

The Drawing or Diagram will update automatically (Test by pressing F5 or Refresh the page), there is no need to publish it again.

Document Link:

Embed code:
The Google Drawing can be Embeded by adding the embed code to your html pages.
The height and width can be adjusted, google site will also give you the 3 standard sizes Small/Medium/Large in the drawing scaled proportions.
<img src=";w=800&amp;h=520">

Google Diagram: