If you're making a slide show, are you just dropping in the photos and stretching them inside the slide show app? Or... are you doing that, checking to see the actual H/W (in pixels) of the final sizes you want, taking the original photo files and resizing them to the final size you need and dropping those (smaller) files into the slide show? That last way will make the slide show load faster and the total slide show file size smaller. Also change the DPI to 72dpi as that's plenty for web-side shows (again a way to make the final slide show smaller for sharing, loading, and backing up).
IE: An untouched photo from a modern camera phone can often be 4Megs each. Meanwhile a resized and compressed JPG of the same photo can often be under 20K. FYI 1 Meg = 1024K so that's a HUGE savings.
(Trust me... a little work up front will make a MUCH better final product.)