You can't have invisible background. You can though, set the background to
black. To do so, click Format > Background. Click on the dropdown, select
black. Then click Apply to all.

I would strongly urge that you don't use the photo album feature.  Your
images will be uneditable in PowerPoint directly.  PowerPoint brings your
pictures in as fills for autoshapes.  The image itself will later need to be
'extracted' again as a picture, which is annoying if there are a lot of
them.  I regularly use Image Importer Wizard.  See Shyam Pallai's website.
This is a really good add-in.  The images are preserved as such in the
presentation.