test: add rectangular test image

WHY IS THIS CHANGE NEEDED?:
- a case in which x and y dimensions are unequal is better at revealing
  dimension storage/conversion issues

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- tile existing image to size x = 1800, y = 1200

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- filenames have changed

EVIDENCE THAT COMMIT WORKS:
- doesn't seem to break existing tests, specifically test_image.py and test_tiler.py