Add alt text to img

footer.php

@@ -82,7 +82,7 @@ $pb_network_contact_form = get_option( 'pb_network_contact_form' );
 			</div>
 			<div class="footer__pressbooks__social">
 				<a class="facebook" href="https://www.youtube.com/user/pressbooks" title="<?php _e( 'Pressbooks on YouTube', 'pressbooks-aldine' ); ?>">
-					<img class="youtube-link" src="<?php bloginfo( 'template_directory' ); ?>/assets/images/yt_icon_mono_dark.png">
+					<img class="youtube-link" src="<?php bloginfo( 'template_directory' ); ?>/assets/images/yt_icon_mono_dark.png" alt="YouTube">
 					<span class="screen-reader-text"><?php _e( 'Pressbooks on YouTube', 'pressbooks-aldine' ); ?></span>
 				</a>
 				<a class="twitter" href="https://twitter.com/intent/follow?screen_name=pressbooks" title="<?php _e( 'Pressbooks on Twitter', 'pressbooks-aldine' ); ?>">