how to add img url to apex css style

I am trying to add a background image to a tab in my salesforce Account tab. Below is the code that i am using. Active should give me an url to image that looks like this.

Which should be the background image of the tab inside the Account tab.

Please advise,

Darko

<apex:page standardController="Account" showHeader="true" 
      tabStyle="account" >
   <style>
      .activeTab {background-color: #236FBD; color:white; width: 170px; height: 40px; padding-top: 25px; font-size: 16px; font-style: bold; border: 0px;
         background-image:url(/servlet/servlet.FileDownload?file=01520000002Z3Ly);}
      .inactiveTab { background-color: lightgrey; color:black; width: 170px; height: 40px; padding-top: 25px; font-size: 16px; font-style: bold; border: 0px;
         background-image: none}
   </style>
      <apex:pageBlock title="Hello {!Account.Contacted_by__c}!" mode="edit" >
        You are displaying values from the {!account.name} account and a separate contact
        that is specified by a query string parameter.
    </apex:pageBlock>
   <apex:tabPanel switchType="client" selectedTab="tabdetails" 
                  id="AccountTabPanel" tabClass="activeTab" 
                  inactiveTabClass="inactiveTab">   
      <apex:tab label="Details" name="AccDetails" id="tabdetails" style="background-color: white; ">
         <apex:detail relatedList="false" title="true"/>
      </apex:tab>
      <apex:tab label="Contacts" name="Contacts" id="tabContact" >
         <apex:relatedList subject="{!account}" list="contacts"  />
      </apex:tab>
      <apex:tab label="Opportunities" name="Opportunities" 
                id="tabOpp">
         <apex:relatedList subject="{!account}" 
                           list="opportunities" />
      </apex:tab>
      <apex:tab label="Open Activities" name="OpenActivities" 
                id="tabOpenAct">
         <apex:relatedList subject="{!account}" 
                           list="OpenActivities" />
      </apex:tab>
      <apex:tab label="Activity History" name="ActivityHistory" id="tabActHist">
          <apex:relatedList subject="{!account}" list="ActivityHistories" />
      </apex:tab>
      <apex:tab label="Notes & Attachments" 
                name="NotesAndAttachments" id="tabNoteAtt">
         <apex:relatedList subject="{!account}" 
                           list="CombinedAttachments" />
      </apex:tab>
   </apex:tabPanel>

</apex:page>