Error when creating a receipt for ProSites


I'm facing a problem when I need to send a receipt for ProSites payment.

I successfully integrated ProSites and PayPal sandbox, but after the payment, when I click on "Send" button to send a receipt copy to my e-mail, I get the following error message:

TCPDF ERROR: [Image] Unable to get image: admin

I'm running a Ubuntu 12 box on AWS. I already installed curl and tcpdf (apt-get install php-tcpdf and apt-get install php5-curl). Also I already checked folder permission and ownership (Apache is running as www-data and WP directory -and its subdirectories- owner is the same).

Any idea?