From my recollection, you need to use type: 'custom:config-template-card' to template the image field or use card mod.
Look here
I would suggest an alternative mehod. Create a sensor template that references the image and state of the text input helper.
An example
text_input:
friendly_name: "Text Input"
unique_id: text_input
value_template: |
{% set level = states('input_text.test') %}
entity_picture_template: >-
{% set level = states('input_text.test') %}
{% set path = "/local/pics/" %}
{% set ext = ".png"%}
{{[path,level,ext]|join("")}}
I created a 1.png and a 2.png and the image of sensor.text_input changes based on the state of the input_text.test
