If we really think about it, every photograph we create is a self-portrait. Notice I use the word create. Every image in which we as photographers put time and effort into creating, not simply clicking wildly, but with intention and deliberation, those images reflect an aspect of our own inner nature.
Whether we choose to photograph a landscape, a seascape, another person, or even an actual self-portrait, we can't help but put our own sensibilities, our own unique selves, into the image by the simple choices we make in framing, exposing, and processing our images. When you look at the bodies of work by such artist photographers as Richard Avedon, Irving Penn, Arnold Newman, Joyce Tennison, Man Ray, and Jerry Uelsmann, among others, you see each of their unique imprints on their images, their own individuality somehow shows through, and in so doing, that quality gives us a little self-portrait of each artist.
Now, in discussions with other photographers, when it comes to this topic, invariably someone will chime in with: "But if one-hundred photographers are all gathered in front of Delicate Arch, in say a landscape workshop, how is each image a self-portrait when you end up with essentially one-hundred nearly identical images?" My answer is that that is precisely the place that those one-hundred photographers are in. They may be in the place of their photography where they are all doing nearly the same image. They haven't yet reached the place of individualizing their images.
But that place is, in a way, a self-portrait "at-that-time" of where you are in your self-realization. Without getting too philosophical, where you are in your life is reflected in your images. I remember some of my earliest images on film, and they were a reflection of where I was then. My images now are vastly different, but there is always and element of me in all that I do. I think if we are honest with ourselves, and in our images, when we look at the vast body of our life's work we will see a changing, growing, deepening self-portrait emerging.