Bootstrap hidden-sm-down not working

Actually, hidden-sm-down won't work with Bootstrap 4 and above
(there use d-none instead of hidden-sm-down, and use d-sm-none instead of hidden-sm-up, see also understanding-details).

With BS4, display utility classes are completely changed. Use this format instead;

.d-{breakpoint}-{value}

More information ; https://getbootstrap.com/docs/4.0/utilities/display/

As Jaqen said, if you use Bootrstrap 3, you should use hidden-sm instead.

Also, if you want to hide the image on small screen, you have to add hidden-xs.

Here's a JsFiddle : DEMO