did you found that sometimes it hang the php when imagecreatefromjpeg() run on bad JPEG. I found that this is cause by the JPEG file U used dont have EOI (end of image) 
the FF D9 at the end of your JPEG

JPEG image should start with 0xFFD8 and end with 0xFFD9

// this may help to fix the error
function check_jpeg($f, $fix=false ){
# [070203]
# check for jpeg file header and footer - also try to fix it
    if ( false !== (@$fd = fopen($f, 'r+b' )) ){
        if ( fread($fd,2)==chr(255).chr(216) ){
            fseek ( $fd, -2, SEEK_END );
            if ( fread($fd,2)==chr(255).chr(217) ){
                fclose($fd);
                return true;
            }else{
                if ( $fix && fwrite($fd,chr(255).chr(217)) ){return true;}
                fclose($fd);
                return false;
            }
        }else{fclose($fd); return false;}
    }else{
        return false;
    }
}