Mixed content warning due to photos being posted with “https”

Some of the recent posts on my site (SSL) are being flagged on Chrome as not secure due to images with http:// URLS (rather than https://). I added SSL years ago, so the find/replace trick likely wouldn’t be a permanent fix. The issue only seems to be happening with some users’ posts, and I verified with them that they’re using the standard upload/add media method. Any ideas what may be causing this and how I can fix it?

Edit: I’m using DigitalOcean hosted on Cloudways with Cloudflare enabled.