[Plugin: NextGEN Gallery] Different backgrounds and sizes for slideshows

Hi, I’m using different backgrounds for my horizontal and vertical slideshows. I know how to set the sizes for the two kinds but how to specify which class (with namely background and other parameters like padding and margin) to use for each ?

I use WordPress 3.4.1 and NextGEN Gallery 1.9.5

Thank you for any help

https://www.ads-software.com/extend/plugins/nextgen-gallery/