That issue is caused by DualImage.php having $imgBaseDir = './public_html/forecast/icon-templates/'; # directory for png image sources WITH trailing '/'
which should read $imgBaseDir = './forecast/icon-templates/'; # directory for png image sources WITH trailing '/'
Keep in mind that for FTP only, the public_html/ directory has to be used. On the website itself, it's always ./ (with no public_html) as the webserver uses the public_html/ directory as the document root.