Many payment gateways, in particular credit card processors, require the HTTPS protocol to be used on your site. In order to activate HTTPS, a valid SSL certificate must be installed on your site. You can purchase SSL certificates from your hosting provider. Your SSL reseller and/or hosting provider can guide you on the correct installation of the SSL certificate on your site.
Unsecured Error Messages
If you had the certificate installed before you installed your WordPress, you may already have set your site to use the HTTPS protocol. Great news for you as your images will be served over HTTPS automatically; however, if you installed your WordPress and then installed the SSL certificate, you are probably using the HTTP protocol. This can create an SSL unsecured message “It looks like there is an image being served from HTTP instead of HTTPS” and this will prevent your site from displaying the green padlock / secured status.
There are a couple of things you can try to fix this: