Import image with migrate tools changing its name
Two options:

_image_fixed:
  - plugin: str_replace
    source: imageName
    search: "jpg"
    replace: "png"

source_full_path:
  - plugin: concat
    delimiter: /
    source:
      - constants/source_base_path
      - @_image_fixed


Or:

source_full_path:
  - plugin: concat
    delimiter: /
    source:
      - constants/source_base_path
      - imageName
  - plugin: str_replace
    search: "jpg"
    replace: "png"