First you need to make sure you are using the latest version of Easy PDF Invoice extension. If you haven't installed it you can get it from magento connect: http://www.magentocommerce.com/magento-connect/site-management/billing-invoicing/easy-pdf-invoice-4072.html
Easy PDF Invoice extension does not support for adding thumbnail image to your invoice PDF. You need to install an addition extension Easy PDF Invoice - Custom Variables to allow adding thumbnail image to your PDF.
You can download this extension from magento connect via this link : http://www.magentocommerce.com/magento-connect/catalog/product/view/id/16526/
After you install the extension you will see new submenu item called Manage Custom Variables
Click to Add Variable
Set the Variable Name as you want, type is Product Attribute and Attribute is Thumbnail then click Save And Continue Edit.
After you save the variable you will see the implementation code.
Copy and paste this code to your PDF template like the image below.
Let see the result:
