[theOZer]

Here's a Tag Search that shows both possible situations.
1) Thumbnails are displayed for threads with a thumbnail attachment in the 1st post.
2) Default Thread Icon displays for threads without a thumbnail in the 1st post.

As I also note in 02 JUL 2008 Post # 175
"And, for what it's worth, you can tweak the Thread Thumbnail Search plugin, have it use a different temp true/false variable, e.g., $tt_displaythumbs_search, and add an additional IF statement to handle regular searches like I'm handling Tag Search."

Now my New Posts and regular searches also are displayed this way, and only my thumbnailed forums show the 'No Image' icon if there's no thumbnail pic.
~OZ