Remove rounded corners of Posters in Arctic Fusion

Hi there,
I would like to remove rounded corners from posters in panels within the Arctic Fusion theme from Jurialmunkey. I found an older post from him saying he uses textures with rounded corners using alpha and then he applies transparency using a colordiffuse. I couldn't find it in the xml files myself yet and didn't get any reply elsewhere, unfortunately.
I'd appreciate any help. Thanks in advance.