! having a text variable name be gif with contents of png is lazy :P

Themes/default/ManageSmileys.template.php

@@ -489,7 +489,7 @@ function template_editicon()
 						</dd>';
 	echo '
 						<dt>
-							<strong><label for="icon_filename">', $txt['smileys_filename'], '</label>: </strong><br /><span class="smalltext">', $txt['icons_filename_all_gif'], '</span>
+							<strong><label for="icon_filename">', $txt['smileys_filename'], '</label>: </strong><br /><span class="smalltext">', $txt['icons_filename_all_png'], '</span>
 						</dt>
 						<dd>
 							<input type="text" name="icon_filename" id="icon_filename" value="', !empty($context['icon']['filename']) ? $context['icon']['filename'] . '.gif' : '', '" class="input_text" />

Themes/default/languages/ManageSmileys.english.php

@@ -92,5 +92,6 @@ $txt['icons_new_icon'] = 'New Message Icon';
 $txt['icons_location_first_icon'] = 'As First Icon';
 $txt['icons_location_after'] = 'After';
 $txt['icons_filename_all_gif'] = 'All files must be "gif" files';
+$txt['icons_filename_all_png'] = 'All files must be "png" files';
 $txt['icons_no_entries'] = 'There are currently no message icons configured.';
 ?>