Issue in switching from not SSL to SSL

Hi, my website have SSL active only in log in area and SSL not forced in visitor area but website is also available under SSL if user put https before.

Now I have set a loading image that must start with http:// or https:// I want the image is loaded in http when SSL is not set and https when user load the website with https because now my site show warning message with SSL because image of the logo in the loading is http.

I don’t want put https because some old client who not support SSL will have issue on see the image on uncopatibile SSL device. I tryed to link the image address :// but this is not accepted i must put https:// or https://

https://wordpress.org/plugins/loading-page/