If you wish to capture the jpg data into a variable, rather than outputting it or saving it into a file (perhaps so you can put it in a database), you might want to consider output buffering. Something along these lines should work:
<?php
ob_start(); // start a new output buffer
imagejpeg( $newimage, NULL, 100 );
$ImageData = ob_get_contents();
ob_end_clean; // stop this output buffer
?>