Adding Image to right side of header – child theme needed?

I have an issue and a question. I found a thread that shows how to add a logo to the right side of a header. I have a bottom menu.

I did this: open helper-main-menu.php file and find
echo avia_logo(AVIA_BASE_URL.’images/layout/logo.png’, $addition, ‘strong’, true);
and add your image right below it as following
echo “<div class=’header-custom-image’></div>”;

The image only displays when it is at around a tablet or mobile size, not at desktop, and it doesn’t look right.
Someone had done this for me before and they created a child theme.

Is that necessary and what would happen if I don’t do that and an enfold update comes out.