修改文件：source\class\forum\forum_upload.php

查找代码：

			if($_G['setting']['thumbsource'] || !$_G['setting']['thumbstatus']) {
				list($width) = @getimagesize($upload->attach['target']);
			}

添加代码：

			if($_G['setting']['watermarkstatus'] && empty($_G['forum']['disablewatermark'])) {
				if (!class_exists('image')){
					require_once libfile('class/image');
				}
				$image = new image;
				$image->Watermark($upload->attach['target'], '', 'forum');
				$upload->attach['size'] = $image->imginfo['size'];
			}

以上方法在x3.1中测试成功。