Associate one item of content with one taxonomy term
It's possible, but not terribly easy in D6.

In Drupal 6 (unlike 7 and 8) vocabularies were not fieldable, so the approach you took is a reasonable one.  You probably do not need a relationship in the view, just adjust the filter to include content types of the new type, and then use sorting to move the to the start or end of the result set. You could sort by content type or use the usually ignored "sticky" field on the nodes if its not in use (set all your new images to be sticky and then sort the list to place them where you want).

You could also create a block that uses the same filter pattern as the original view, but limit to the new content type and the only 2 results. Then place that block on the page that's displaying the grid.

Note: Drupal 6 is no longer officially supported by the community. While there are some places that are offering long term support for Drupal 6, you should plan to move to Drupal 7 or 8 soon.