Image als Hover (div css/html over Nextgen gallery shortcode) wilt niet werken!!

Currently I have the latest version of NGG on my WP site.

I want to place a image as hover on a ngggallery (inserted by shortcode). Can this be done by editing the NGG settings (so in the future all portfolio image will hover automatically with my own pattern?) or does this need to be done by HTML&CSS.

See this link for an example of my needs (mouseover on a portfolio image):
http://justcreative.com/portfolio/#all

See this link to view my current website:
http://stuch.nl/portfolio

Any guru around?

Sincerely,
Ko-Lin
Stuch.nl

https://wordpress.org/plugins/nextgen-gallery/