Post Grid pulling images at 300×200

Hello, we're using the Post Grid plugin, and it seems as if the plugin is pulling images at 300px wide.  Is there any way to change this?  It's not an issue to go into the plugins code, I'd just need to know what to change and where.  We'd like these images to be 600px wide to not lose resolution, they look very grainy when we resize them bigger with CSS.