thanks for all of your efforts neilo, it's looking better.
i tried commenting out the following lines in gallery.js (lines 57-59, in the slideshow blinds js folder).
i also edited gallery.js in the slideshow dissolve folder.
// this.outerWrapper.setStyle('width', this.wrapper.getSize().x + 'px');
it now seems to leave the width at 960px
slideshow dissolve seems to look ok
slideshow blinds moves over to the left, but the width remains at 960px :-)
i haven't tested a slideshow with different sized images yet, so there may be other implications