This is how I've landed to reduce image file sizes from the command line. From what I understand gopher and gemini are text based protocols so this space is probably not really appropriate for an inundation of images, but its a handy tool to have for other projects. I will be using the ImageMagick convert tool.
Quick tutorial from DigitalOcean
To install:
$ sudo apt install imagemagick
This command reduces the image size to 400 width and makes it gray-scale:
$ convert mountain.jpeg -resize 400 -colorspace Gray mountain-sml.jpeg
The result:
$ du -ah 2.6M ./mountain.jpeg 28K ./mountain-sml.jpeg
alt text: b&w photo of mountain range taken from below in grassy tussock, next to a flowing creek on the right.
2.6MB to 28KB. Brilliant.
This is another groovy web based alternative that popped up in my feed the other day. Dithering gives it a bit of an artsy feel:
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