diff --git a/wp-admin/options-media.php b/wp-admin/options-media.php
index 9367cb8b6f..bbc3a3e84d 100644
--- a/wp-admin/options-media.php
+++ b/wp-admin/options-media.php
@@ -49,7 +49,7 @@ include('admin-header.php');
 </table>
 
 <h3><?php _e('Image sizes') ?></h3>
-<p><?php _e('The sizes listed below determine the maximum dimensions to use when inserting an image into the body of a post.'); ?></p>
+<p><?php _e('The sizes listed below determine the maximum dimensions in pixels to use when inserting an image into the body of a post.'); ?></p>
 
 <table class="form-table">
 <tr valign="top">
diff --git a/wp-includes/formatting.php b/wp-includes/formatting.php
index f0c8d7e1e6..a9c396b469 100644
--- a/wp-includes/formatting.php
+++ b/wp-includes/formatting.php
@@ -1820,6 +1820,12 @@ function sanitize_option($option, $value) {
 			$value = sanitize_email($value);
 			break;
 
+		case 'thumbnail_size_w':
+		case 'thumbnail_size_h':
+		case 'medium_size_w':
+		case 'medium_size_h':
+		case 'large_size_w':
+		case 'large_size_h':
 		case 'default_post_edit_rows':
 		case 'mailserver_port':
 		case 'comment_max_links':