Slideshows allow you to share various presentations with other users in the 3D world. They accept common formats like PowerPoint and PDF, and they will also process groups of separate image files, turning them into a PDF and a single presentation. The image below shows the current usage guidelines:
As you can see, the slideshow board accepts files from your hard drive, or via URL. You cannot post a stack of image files via URL however, only single files. In addition, URLs must end in a valid file format, such as PDF or PPT (there are a wide variety of supported file formats). After you or another user have successfully uploaded a presentation, it should be possible to download it in PDF format by clicking the disk button with the green arrow on it.
Also, see the page on FileChoosers for guidance on selecting files.
You can delete, drag and resize many slideshow boards if you enable laser (hit L) and use the corner handles.