If anything could potentially get some level of compression of already compressed images I would expect lrzip [1] if you have large enough of an archive but there really isn't anything designed specifically for your use case as far as I know. So maybe put a large number of images into a .tar file and then use lrzip on that .tar file. It would need to be a very large archive. In the past I gained about 3% to 5% on large image archives using 7-zip but it was really slow and not worth it for me which says something as I am very patient.

There is potentially some compression magic that could be performed by transforming images using ImageMagick [2] or GraphicsMagick and then compressing them but that gets into the topic of potentially reducing image quality. Not compressing them with those tools but rather confining color pallets, depth and a few other variables to optimize the images to be more batch compressible for marginal gains. If you are not concerned about image quality then those tools can absolutely compress images without any trickery. resize, adding smoothing or blurring, etc...

    for Derp in *.jpg; do convert -resize 30% "$Derp" "resized-$Derp"; done

This is a topic people could debate until the end of the universe so instead give lrzip a shot if you have the time and CPU resources. If you get more than 5% please let us know. Otherwise one could use ImageMagick to batch resize, blur or other of the image files at the risk of losing image quality.

[1] - https://wiki.archlinux.org/title/Lrzip

[2] - https://imagemagick.org/index.php

Wow, thank you so much, Bender16 — your comment feels like an article on its own! I really appreciate the depth and clarity you shared. It’s rare to see such thoughtful technical advice laid out so well.

I’ll definitely experiment with lrzip and the .tar approach — and I hadn’t considered pre-processing images with ImageMagick that way. Your tips opened up some new ideas I hadn’t even thought of!

So like HEVC/.h265?

Compression works because the data is structured and ordered. A folder of photos is neither. When you do structure and order your photos you have, well, a video.