Hi and welcome to elkarte.
The first things to check are:
- Forum URL: ACP > Configuration > Server Settings > Database and Path > Forum URL (make sure is https://domain.com)
- Theme settings: ACP> Configuration > Theme Management > Theme Settings (mage sure to use https)
- Avatar settings: ACP> Forum > Attachments and Avatars > Avatar settings (make sure to use https)
If you still see the grey padlock in your browser (instead of green) you may check into your browser to see which resources are loaded over an http connection.
Once you fixed this you may also install one of these 2 add-ons to cache all images inserted with IMG tag that are not hosted on a secure connection:
https://addons.elkarte.net/enhancement/ExtImageProxy.html
https://github.com/Spuds/Elk_Image_Cache_v10