I've put together a new plugin for converting images to HTTPS. When you switch your site over to HTTPS, chances are you have many images linked from previous posts, either to your own site or external sites. But linking to images using the http:// protocol instead of https:// causes "mixed-content" warnings in most browsers, and the browser may now say the page is not secure.
Some image links can be rewritten to HTTPS where supported, but not every site supports HTTPS. This plugin handles both those situations!
HTTPS Image Proxy plugin
If an image is linked insecurely, the plugin will check a list of known secure hosts and change the URL to https:// if possible. Otherwise, it will change the URL to use a proxy script on your own site, so the image appears secure to the user. The server fetches the original image and caches it (if a writable directory is specified).
Note: this plugin is currently in beta so it may be unstable, but feel free to give it a try and let me know how it goes. Make sure to follow the instructions in the README, as manual configuration is required (via a config.php file rather than Q2A admin page).