Menachem Shmei: I’m following up here, since you linked to this thread for what I inquired.
Firstly, I believe “Gedolim pictures” more or less became a thing outside of the Chasidish velt first. The Chasidish Rebbes were mostly originally opposed to being photographed. And the Yeshivish velt started getting into it much earlier. As you mentioned, even the Lubavitcher Rebbe was initially not in favor of it. Although, the opposition was much stronger by others such as Satmar, etc.
If the argument in favor is that “thinking of a tzaddik’s image as an important method to yiras shamayim”, then that should support, as well, erecting life-like sizes statues of Tzadikim, using their real-like image. Imaging walking to Shul, and right outside in the Shul yard is a life sized statue in the image of the local Tzadik.
In any event, I’m not so much questioning having pictures of Tzadikim. Today that is found across all sectors, Chasidish, Yeshivish and Sefardish. What I am trying to understand, and you seem knowledgeable, is why in Lubavitch (almost exclusively) we find these huge life sized images of the Rebbe at every general venue, like wedding halls, on cars and trucks, and on many other facilities, buildings and structures. The size, places, frequencies and ubiquitousness seems rather unique.
Also, it’s only the last Rebbe. Why not any of the previous Lubavitcher Rebbes and/or other Tzadikim? In other Yiddish places with pictures you’ll generally find those of multiple Tzadikim.