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:
LR never changed the original image. It records the edits you do in the catalog file and in a XMP file if you have that option checked in the LR and or catalog settings dialogs.
To get a copy of an image that you have applied adjustments to with all those adjustments burnt in you need to Export it.
Export them (the edited jpgs) to any place, most convenient is a folder on your desktop. Call it Slide Show Prep Stop. Drag the images in the desired order from that SSPS folder to the app; once the slide show is set up in final form you can deep-six the SSPS