Very, very nice! :)
Pull those style tags out and place them into a file - then include the file within your php file.
// Add CSS
$document =& JFactory::getDocument();
$document->addStyleSheet( JURI::base() . 'plugins/content/slimbox.css', 'text/css', null, array( ' id' => 'StyleSheet' ) );
These two files can be copied into a folder named magazinelayout:
Usually, if it's just the php, xml, css and 1 js file, they can all stay in the root of the content plugins - but if there is more than that, creating a folder can be helpful. You could add your css to that folder, if you choose.
This is a beauty - there is no doubt. What a great way to take a set of up to eight images and turn it into a perfectly arranged gallery. This has the potential of dressing up many J! sites.
I love how you allow the image manager to be used. You need to consider, though, that a lot of HTML is going to slip in there between the images. When that happens, the site bombs hard.
When we used a NoWYSIWYG editor - it only recognized the first image - experimenting more, it appeared that paragraph tags around the images created a perfect feed to your program.
Give it some thought how you can handle the inexperienced user with the editor - and the noWYSIWYG editor. At bare minimum, make it perfectly obvious in the instructions what that HTML should look like inside of the plugin.
This is a wonderful, wonderful, wonderful plugin. People are going to love it. Thanks for taking the task. We'll look for your next post!