I think you may be looking at the "Pblog" style sheets, instead of the standard ones. To change over to it, go into your gallery edit page and look for style sheet type. Change it from original, to Pblog then click update. Once you've updated, go to the style sheet drop down menu, and you'll see the various Pblog styles you can use. Pick one, and update again. Now you'll have only one size photo showing down the page, without thumbnails.
Hope this does it, more info in the "help" section, here:
http://www.pbase.com/help/stylesheets