You might wonder what the difference is between a photography theme and a portfolio theme. The truth is that at times they might both serve the same purpose. But at other times they may be a little different.
To my mind, a portfolio theme concentrates on providing a section of the site that is dedicated to showing off a collection of images.
Of course a photography theme might do that too, but the way I look at it, a photography theme puts more emphasis on the blog aspect and less emphasis on creating a separate section to show off a collection of photos.