Drop your own images in this folder. Every page falls back to
placeholder.jpg automatically if a named file is missing, so the site
still works before you add anything here.

Series images the seed data expects (sql/schema.sql):
  2-series.jpg
  3-series.jpg
  4-series.jpg
  5-series.jpg
  6-series.jpg
  7-series.jpg
  8-series.jpg

Individual models all currently use placeholder.jpg - when you add a
model through the admin area you can rename an uploaded photo to match
and update the "image_file" value for that row (e.g. via phpMyAdmin),
or extend add_model.php / edit_model.php to accept a file upload if you
want that to happen automatically.

Recommended size: roughly 800x500px, JPG or PNG, kept under ~300KB each
so pages stay quick to load.