open document tab,and image what u want to display,right click on image copy and paste it in `<apex:image >`


    <messaging:emailTemplate subject="vf template" recipientType="Contact" >   
       <messaging:plainTextEmailBody >
        Congratulations! This is your new Visualforce Email Template. 
        <apex:image url="sahitya0907-dev-ed--c.ap2.content.force.com/servlet/…; /> 
    </messaging:plainTextEmailBody> </messaging:emailTemplate>