Neutica Thumbnail Image Size

Dear Supporters,
I recently have been using the Neutica Theme from Allan Cole. I´ve made my slight changes on the theme. But I haven´t managed to change the thumbnail size yet. It is set to 185x185px but I need to increase to 185x225px. Everytime I change the layout.css the images get streched to the new size. It might has to do with a size-overwrite or something like this, but unfortunately these are the things, I don´t know anything about.

Is there anyone who could spare some of his precious time to help me?

Many thanks, SGT