Unlike all the other icon files the open.gif, closed.gif and item.gif used in the admin tree are not used if present in the pix folder under the used theme folder.

The lines that seem to be causing this are in blocks\admin_tree\block_admin_tree.php:

38: $this->tempcontent .= '<span id="vh_span' . $this->spancounter . 'indicator"><img src="' . $CFG->wwwroot . '/blocks/admin_tree/open.gif" border="0" alt="[open folder]" /></span> ';

63: $this->create_item($content->visiblename,$CFG->wwwroot.'/'.$CFG->admin.'/settings.php?section=' . $content->name,$CFG->wwwroot .'/blocks/admin_tree/item.gif', $class);

68: $this->create_item($content->visiblename, $content->url, $CFG->wwwroot . '/blocks/admin_tree/item.gif', $class);

145: $this->content->text .= ' getspan(spanid + "indicator").innerHTML = \'<img src="' . $CFG->wwwroot . '/blocks/admin_tree/open.gif" border="0" alt="'.$strfolderopened.'" />\';' . "\n";

149: $this->content->text .= ' getspan(spanid + "indicator").innerHTML = \'<img src="' . $CFG->wwwroot . '/blocks/admin_tree/closed.gif" border="0" alt="'.$strfolderclosed.'" />\';' . "\n";

157: $this->content->text .= ' getspan(spanid + "indicator").innerHTML = \'<img src="' . $CFG->wwwroot . '/blocks/admin_tree/closed.gif" border="0" alt="'.$strfolderclosed.'" />\';' . "\n";

163: $this->content->text .= ' getspan(spanid + "indicator").innerHTML = \'<img src="' . $CFG->wwwroot . '/blocks/admin_tree/open.gif" border="0" alt="'.$strfolderopened.'" />\';' . "\n";