I haven't used Powerpoint. With Keynote you open a presentation, add images to the side bar on the left, edit them from the sidebar on the right, and then File/Export to/Movie and drag the slideshow movie into the timeline of an iMovie project. A nice thing about Keynote is that you can add all kinds of actions to the slides from the Animate sidebar, and create interesting animated titles. Once you export the slideshow movie and drag it into iMovie you can adjust the speed with the speed adjustment feature in iMovie. That is, Control-click on the movie clip and select Show Speed Editor in the drop down menu. That will reveal a speed adjustment slider handle in the upper right hand corner of the clip. Slide to the left to speed up, and to the right to make the clip slower.
I believe that keynote has its own templates. I am not aware of any third party ones.
Another way would be to use iMovie to create the slide show. To do that, drag the images into an iMovie project timeline and arrange them in the order you want. You can apply all of the effects available in iMovie, such as the Ken Burns effect for zooming and scanning, cropping, titles, transitions, and themes. You can easily adjust individual slides as to duration, that would be more difficult if working with a slideshow movie.
Another way is to make the slide show in the Photos app and then export it as a movie to your desktop. Then drag the slideshow movie in the timeline of an open iMovie project.
-- Rich
