Slideshow to include whole gallery not just page

Have galleries that need to accommodate anything from 60 to 260 photos. Have currently set gallery to display 15 pictures a pages. Problem here is that if customers click on slideshow it only shows pictures from one page and not the whole gallery and they get confused. Could put all pics on page but this not desired option. Is it possible to make the slideshow display every picture in the gallery rather than those on an individual page? If so, what modifications would I need to make? Any help much appreciated.