> INSERT INTO image_test
> VALUES (NULL, "
https://s3.amazonaws.com/myimagefolder/image1.JPG";);
Get rid of the first semicolon.
> header('Content-type: image/JPG');
That is used if the PHP script is generating the image, but you don't have the image; it is elsewhere.
When building the html page, use no header (or one that is appropriate for html). Then include in the html either of these:
echo "<img src='$row['image_url']'>" -- without the semicolon!
<img src="<?php echo $row['image_url'] ?>">
Personally, I prefer:
$url = $row['image_url'];
echo "<img src=\"$url\">";
Any of the echos will lead to the _browser_ imbedding the image in the page.