It looks like you downloaded Power Gallery 1, not power gallery 2.
In Power Gallery 2, there is an option in the gallery admin to edit the gallery settings and change the image sizes.
changing the CSS does not change the image size that is used for the thumbnails.
By default the thumbnails will be created at 60 pixels, looks like you change the CSS to display the thumbnails at 80 pixels, this is just enlarging the 60pixels images that where created, you need to use the gallery admin to change the settings for the image size for that gallery.