I like your idea of using art postcards and having a VTS poster! I found this link to The Learning Network at The New York Times that puts a picture up each Monday for students to analyze and discuss.
https://learning.blogs.nytimes.com/2015/01/05/whats-going-on-in-this-picture-jan-5-2014/?_r=0I was also looking for a little more direction in HOW to teach VTS to my students and found this link with some useful information.
https://www.educationworld.com/a_lesson/teaching_visual_thinking_strategies.shtml