[Plugin: jQuery Colorbox] Opening 2 lightbox With Nextgen

Hey guys,

I´m using colobox and nextgen, I would like to use colorbox to all site images, but not for Nextgen galleries.

I´ve been googling and found this:
http://ww.wp.xz.cn/support/topic/plugin-jquery-colorbox-feature-request?replies=4
“If the CSS class “colorbox-off” is assigned to the images”

http://ww.wp.xz.cn/support/topic/plugin-jquery-colorbox-using-colorbox-only-in-posts-and-pages-nextgen?replies=2
“you’d have to add the CSS class “colorbox-off” to the links containing the images”

But, i´m kind noob on code matters so, my question is

How to add CSS class “colorbox-off” to the images?

Thanks in advance