Your original imported images always remain unchanged, completely in their original pristine state as they were when they were imported from the camera. All of the changes are stored in the catalog in Lightroom. The benefit of this is that it ensures the integrity of the master images. For the slideshow you are trying to create, you need to export copies, probably JPEG images, and probably smaller than the original files. In the export dialog you can set up all of your export parameters which include the size of the exported images as well as a destination folder of your choice. All you have to do is take the time to completely set up your export options. This short video might shed some light on the subject:
Lightroom CC - Exporting Images - YouTube