Jquery window.send_to_editor function

It is just a little syntax error in your window.send_to_editor function:

imgurl = $( 'img', html ).attr( 'src' );

should be

imgurl = $( 'img', $( html ) ).attr( 'src' );

because your html variable is neither a valid DOM nor a jQuery Element.

Additional Note:
If you want to use the Thickbox on multiple text fields, I’d recommend storing and creating your send_to_editor function:

var image_field,
    store_send_to_editor = null,
    new_send_to_editor = null;

jQuery( function( $ ) {
    store_send_to_editor = window.send_to_editor;
    new_send_to_editor = function(html) {
        imgurl = $( 'img', $( html ) ).attr( 'src' );
        image_field.val( imgurl );
        tb_remove();
        window.send_to_editor = store_send_to_editor;
    };
    $( document ).on( 'click', 'input.select-img', function( evt ) {
        image_field = $( this ).siblings( '.img' );
        check_flag  = 1;
        tb_show( '', 'media-upload.php?type=image&TB_iframe=true' );
        window.sent_to_editor = new_send_to_editor;
        return false;
    } );
} );