Losslessly recompress PNGs to save space

MAINTAINER_BUSINESS.md

@@ -30,3 +30,36 @@ If you already have the dependent library packaged, it should be picked up norma
 It may be a good idea to provide `-DCPM_USE_LOCAL_PACKAGES=yes` to cmake to
 ask CPM to show an error if the package is missing, instead of trying to download
 anything.
+
+## Compression of newly added images
+
+PNG images can frequently be more efficiently compressed by using a tool designed
+for the purpose, such as [pngcrush](https://pmt.sourceforge.io/pngcrush/),
+[oxipng](https://github.com/shssoichiro/oxipng), or
+[ect](https://github.com/fhanau/Efficient-Compression-Tool).
+
+When adding new images to the Stellarium project, contributors should make sure
+that added PNGs (and exported images from SVG sources) are compressed as much
+as is reasonably achievable.
+
+Benchmarks [suggest](https://css-ig.net/benchmark/png-lossless) that `ect` is the
+most efficient recompression tool for full color images, particularly larger ones,
+so that is what we recommend.
+
+### Using ECT to recompress images
+
+ECT is an open source tool that compiles and runs on Linux, Windows, and macOS.
+To compress a PNG image in place, run the following command:
+
+    ect -9 image.png
+
+The `-9` specifies the maximum effort in searching for optimal compression
+parameters, and is not appropriate for use in CIs or build scripts due to its
+low speed. `-3` is the default compression speed, which is much faster and
+compresses almost as well.
+
+Note: when recompressing an entire directory of existing images, it is advisable
+to add the option `--strict`. This prevents `ect` from stripping metadata out of
+images. In rare cases, this metadata could affect how the image is displayed.
+If you did not create an image and cannot verify changes to it, avoid making
+changes to image metadata.