ok, i could find how to do it… if anybody needs it:
in the “admin-functions.php” i add this line:
@ imagerectangle ( $thumbnail, $xstart, $ystart, $xend, $yend, $lineColor);
after this one:
@ imagecopyresampled($thumbnail, $image, 0, 0, 0, 0, $image_new_width, $image_new_height, $image_attr[0], $image_attr[1]);
ref:
$xstart and $ystart are the [left,top] point of the image.
$xend and $yend are the [right, bottom] point of the image.
$lineColor guess what??!! yes the line color.