Visualforce Displaying Image in Pdf

I have come across a scenario image is not appearing in pdf  sometimes.For this I have shared some tricks.
It is very easy to display an image in Visulaforce page.
Process

• Crate a static resource and store that image
• Click in View file link 
• Copy the url
• Add apex:image tag in vf page.

 <apex:image url = "https://ap1.salesforce.com/resource/logo"  width="180" height="60" />

Image will be displayed. When you render that page as pdf then sometimes image will not be appear.

To appear image every time, I have done some work around and I would love to share it. 

Lets talk about the image url --- "https://ap1.salesforce.com/resource/1380178695000/logo".

All must be familiar with each term except this 1380178695000. What is that value?

This is nothing but the lastmodifiedtime of static resource.1380178695000 must be dynamic. To get dynamic value I have used some function SystemModStamp.

Here is my page and Controller.

<apex:page controller="ImageController" renderAs="pdf" >
  <table>
        <tr>
            <td>
              <div>
                 <apex:image value="{!sLogo}"  width="180" height="60" />
                 <!--<apex:image url="https://ap1.salesforce.com/resource/logo" width="180" height="60" />-->
              </div>
            </td>
        </tr>
    </table>

</apex:page>

Controller is 

public with sharing class ImageController {
    public String sLogo {get;set;}
    public ImageController(){
        list<StaticResource> listOfStaticResources = [SELECT Id,SystemModStamp
                                                      FROM StaticResource
                                                      WHERE Name = 'logo'];
                              
        if(!listOfStaticResources.isEmpty()) {
          sLogo = '/resource/'+listOfStaticResources[0].SystemModStamp.getTime()+'/logo';
        
           System.debug('Printing after slogo'+sLogo ); 
        } else {
            //sLogo = (invoiceit_s__Configuration__c.getValues('COMPANY_LOGO').invoiceit_s__String_Value__c);
        }
    }
}

Now image will appear in pdf all time.