I actually fixed this by deleting my slider images, re-uploading them, and setting up the sliders again. I think it has something to do with whether ssl is enabled or not at the time the images are uploaded. But yeah, now my slider images are being loaded via https, so all is good.
I did see a number of plugins that help with ssl installation, so those might have helped as well. I didn’t try any of them, since I solved my problem via a simpler method. Hopefully I won’t run into this again since ssl is enabled on my website for good now.