ChenXiu

Nevermind. I figured it out.
"sprintf()"

Good point. Thank you.
Using Data Base 64, the statistics are fairly humble:
image size: 16K
gzipped page size (using "page info" in browser): 19K
actual page size: 35K
For me, using readfile() seems to be the way to go.
Using this header in readfile, the image can have a tidy filename should a visitor want to right-click/save the image:
header('Content-Disposition: attachment; filename="Whatever_I_Want"');
Further, should the actual image be missing, readfile can display an image of an error message:
$imgData = getimagesize($complete_filename_and_path);
if( !$imageData ) { $file_name = "image-of-an-error.png"; }
Thank you again.
p.s. (even though I solved it before you did 😃 )

Thank you Barand. Appreciation offered.
Fortunately (and unfortunately) I realized that there is no answer to my original question (it can't be done).
QUESTION: Can a "where clause" be added to an "insert.....on duplicate key update" query without additional queries?
ANSWER: NO.
REASON: Because for the "...on duplicate key update" portion to know whether some string exists in some column, there must, by definition, be an additional query.
Although I appreciate Your's and Req's answers because they offer alternative "mysql-based" solutions, I don't like them.
Req's answer of creating additional tables and then having to tie them creates all kinds complexity I don't need in my life.
And your answer would result in an abominably large, and endlessly growing, table.
My solution ended up being that $_SESSION["flavor"] captures vistor's $_POST["flavor"], with the following simple line of code:
if ( strpos( $_POST["flavor"] , $_SESSION["flavor"] ) == FALSE ) { mysqli_query( $conn , $barands_concat_query ); }