As Dave says you will break some images as some hosts to not have https, before doing any drastic rewriting, try adding an
upgrade-insecure-requests header to your headinclude template and when you browse your forum with a compliant browser such as Chrome or Firefox then it will automatically request the https versions and you can get an idea of the impact that rewriting all the posted images to https will have.
Code:
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">