How does one add a photo placeholder to a libreoffice slide
If recall correct what MSO PowerPoint does, and interpret your question right, this could be the answer.
Impress does not differentiate between place holders for text, images, charts, videos and table. Each place holder can be used for one of the elements. See my screenshot:
You only have to select how many placeholders you want and how they should be arranged on a slide. This you can do in the under Properties in the Layout window.
Let me recommend to have a look at the free guides which you can download from http://www.libreoffice.org/get-help/documentation/