Custom base URL does not work for generated images

I am using my site in https and enabled simple xml for the particular content type with images enabled for sitemap. but the problem is that sitemap is still showing http in the urls for images. But the page urls are generating with https. Also I have provided base url as "https://" in XML settings. I tried regenerating and clear cache but still xml file did not fix this problem. Refer the attachement

Thank you.