Ted is apparently standing in front of a standard portrait studio wall that has a rear screen. So is Wylie. It used to be department stores had photography sections were you could get your picture taken, kids pictures taken, etc.
I don't think Ted would have joined the Process. He was painfully shy and a social recluse. He never joined anything in his life. The Process is an interesting group. They are an occult group and a cult. They professed a sort of unity of opposites, citing belief and worship of Jehovah and Lucifer, as well as Christ and Satan. They seem to fit into the wild fringes of the 1960's west coast counter culture. Several of the magazines they published at the time focus on themes and images of sex, death, murder, ritualism and symbolism. Religion is a constant focus, the unity of opposites I mentioned, Christ and Satan.
None of this is stuff that Ted, as far as we know, was interested in. Ted was not religious. He avoided people in general and groups in particular. This is why after two years earning money as a professor at Berkeley he bought a cabin in the middle of the wilderness in Montana. If Ted had joined the Process, or any group, we would likely know about it from his brother, peers, witnesses, records, etc. Ted hated hippies, drugs and the counter culture movement. He believed in individualist primitivist anarchism. He loved living in the wilderness and hunting animals. I don't see anything in the Process that would appeal to him, other than Ted had interest in Norse myths, and some aspects of Norse show up in the Process. But everything else they believed he would have had no interest in or even hated. He did not join groups. Even with the radical environmental group "Earth First!", Ted went to a few meetings but never joined. And that was a group he was mostly in agreement with, until in his mind Judi Bari "emasculated" it by turning it from violent actions to peaceful protest. But even when he was in agreement with them he did not join.
Yes Ted is dressed in black in the picture, though they appear to be ordinary clothes, not robes or a uniform. We don't why Ted had this portrait studio picture taken. That is a mystery. There were some other pictures in his cabin stuff, and we know he had a picture taken for an Illinois gun permit and for a State Department passport. I just don't know what these pictures were for. His brother mentioned once that Ted told him that he was thinking about joining some kind of personals section dating club, in an effort to overcome his shyness, loneliness and self described "acute sexual starvation". It is not clear if he ever went through with it and it seems he did not go on any dates. But maybe when he was thinking about doing it he got these pictures done. Maybe they were for a U Cal Berkeley ID or some other kind of ID. Maybe they were to be part of a ruse to get a false ID of some kind. Or for some other purpose we just don't know.
You and all interested people should read some of the books about Ted. Robert Graysmith actually wrote a pretty good biography of Ted called "Unabomber: A desire to Kill". His neighbor wrote a book about his Montana life called "Unabomber: The Secret Life of Ted Kaczynski", and there is another book about his Harvard years. Also Doug Oswell has a ton of info on Ted at his site
www.unazod.com and in his excellent book you can order at the site or at Amazon.