javascript gallery

Can someone help me fix my code for my javascript gallery. I am new
to javascript and I'm having trouble with this.

I want the previous and next button to be programmed so that not
only the thumb images change but also the large image changes as well.

You can see the code at: http://www.helenezgagowski.com/paintings.html

My files can be downloaded from:
http://wikisend.com/download/447736/01_19_11.zip

Can someone help me do this?
thanks
erica

Thanks for the information. I think the change that I want to make is more involved than my knowledge of javascript so I hope that I get some answers on his web page. Thank you so much