Errors on Google Webmaster Tools Sitemap

Hi, I’m getting the following error message on Google Webmaster Tools for the XML Sitemap “Sitemap contains urls which are blocked by robots.txt.” The url it is referring to is https://www.xxxxx.xxx.uk/wp-includes/js/tinymce/plugins/wordpress/img/trans.gif so is not something that should be listed on the sitemap anyway. Any suggestions about what to do would be gratefully received. Thanks ??

https://www.ads-software.com/extend/plugins/google-analytics-for-wordpress/