BD is interlaced, photo's are not.
you can check Always Deinterlace in the Field Options (right click on the photo)
How did you create the slideshow, in Premiere Pro or Encore? (Create them in Premiere Pro)
If i do slideshows from photos i always make them @ 1080p24 since it is progressive and supported by Encore when burning a BD disc. I get great results when i do this.