To add an image slideshow to your stream, you'll want to hit the green "Add Layer" button on the left side of the editor, select "Image Slideshow" from the list of layer types. Once you've done this, you'll be dropped into a menu where you can adjust the slideshow settings as well as add/remove images from your slideshow.
To edit your Image Slideshow, click on the slideshow layer in your editor, and then select the "Edit Slideshow" option at the far left of the layers' property bar.