Hi poptimistic,
Thanks for using my plugin. I apologize, but I’m not certain I understand your question. If you’re seeing some odd behavior, you may have to play with the CSS a bit. If you look at line 50 of the CSS file, you’ll see I specify a width variable of the list item. You might have to shrink that to 270 to match your image size.
I’d suggest copying that data into your own arconix-portfolio.css
file and saving it in the root of your theme folder. That will make sure you won’t lose your changes when I update the plugin.