To generate Google Charts (images) in Visual force-PDF

To enable the Images you have to unblock the callout from apex code. To make a callout from apex code you have to specify the end point in the Remote Site Settings as following,

Setup -> Administration Setup -> Security Controls -> Remote Site Settings -> New Remote Site.  

1. Remote Site Name: A name used to refer to the Remote Site Settings.

2. Remote Site URL: A URL of remote site. For example, to enable Google charts:

    “https://chart.googleapis.com”

3. Disable Protocol Security: Check the Checkbox (As the PDF renderer is a code on the org

     making the request and as such gets denied access if you've not expressly allowed it).

4. Description: An optional description of the Remote Site Setting.

5. Active: Check the Checkbox (By default it is checked to make the settings active).