Text under slideshow not staying centered

I am trying to keep the text under my slideshow image centered but it changes positions as the browser window is expanded / contracted :

http://www.stephenholding.com/clear-earth-draft-iii

I researched other forums where I was instructed to add this code to the Page editor:

[portfolio_slideshow centered=true]

Id did this but it does not seem to be working the way it should.

Suggestions?

http://ww.wp.xz.cn/extend/plugins/portfolio-slideshow/