Best Way to Display Posts by Tag (not category)

by

  1. Query first set of posts (probably with get_posts(), unless you feel up to using WP_Query directly).

  2. Loop through it, display, save their IDs in some variable.

  3. Loop through them again, display thumbnails.

  4. Query second set of posts, excluding IDs saved earlier via post__not_in.

  5. Loop through it and display.