HTTPS Security Warning

I switched over a client’s site to https from http but I’m getting a security warning:

“You connection to this site is not fully secure.
Attackers may be able to see images you are looking at on this site and trick you by modifying them.”

I’ve been able to switch over a number of sites without any problems till now. They are all WordPress sites and I’ve been using a plugin called Really Simple SSL. Does anyone know how to fix this problem and get rid of the security warning?

You can see the site that’s having the problem here: