.= adds to a string, = sets a string. You want = in this case. i.e.:
code:
$sold1 = $row[sold];
if ($sold1 == yes){
$soldstring = "<img alt=\"sold\" src=\"./photos/{$row[thumb]}\" />"; }
else {
$soldstring = "<img alt=\"notsold\" src=\"./photos/{$row[thumb]}\" />"; }