Call header() with the right 'mime' type to announce that it is an image -- and you need to specify jpg/gif/...
Also get rid of the <BR>. The body should have nothing but the image.
And this cannot be a part of a page. It needs to be a separate call. That is, the html page will include something like
<img src=http://.../myimg.php?id=123 height=234 width=345>
Your "myimg.php" program does basically: header(), SELECT the img, echo the image.
The height= and width= are a kindness to the user -- it helps the browser be less jumpy. And if the values exactly match the image, then there is less work for the browser.
header('Content-type: image/jpeg');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . strlen($jpg));
echo $jpg;
And... If you have a closing "?>" (not really necessary), be sure not to have a blank line after it.