Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 38688)
+++ src/wp-includes/media.php	(working copy)
@@ -3139,6 +3139,11 @@
 		) );
 		unset( $possible_sizes['full'] );
 
+		// The "thumbnail" size should always be set as it is used in the media modal and the media library
+		// to display the image thumbnails. If it is removed, they will end up using whatever size is available.
+		// For example they will load the full size images (which are far too large) if only they are available.
+		$possible_sizes = array_merge( array( 'thumbnail' => __('Thumbnail') ), $possible_sizes );
+
 		// Loop through all potential sizes that may be chosen. Try to do this with some efficiency.
 		// First: run the image_downsize filter. If it returns something, we can use its data.
 		// If the filter does not return something, then image_downsize() is just an expensive