Hi, stevenbrm.
For the Standard and Image post formats the thumbnail image for portfolio items will be created from the featured image set for the item. You can’t set a slideshow as a featured image.
For the Gallery post format the preview image will be a slideshow of the gallery images, for the Audio post format it’s an audio player, for the Video post format it will be a video player.
Please try to remember to start a new topic in this forum for each separate issue you have rather than tacking on unrelated questions to an existing topic. We find that separating each question into a separate topic keeps the information much clearer and easier to follow. It really helps other users to find information and it prevents threads from wandering down Tangent Lane. If you don’t mind, please always create a new topic for each separate issue that you have.