OK, so we have loved them, made them, used them, now I recommend compressing them.
There are four of options for compressions technologies in the apple PVRTC tool, texturetool:
For realistic looking images, they recommend perceptual, but there’s still a lot of artifacting with that technology.
You might try compressing them with the PVRTexTool by Imagination Technologies.
They have a command line version that run on Mac, Windows, etc. There’s a window GUI that is nice too.
There are some nice things about the PVRTexTool, my favorites being that it let’s you resize the image during the compression and works on non-square images.
The resize feature of the PVRTexTool came in handy for my last project. The only way I could get decent looking compressed images was to doubled the texture atlases in Photoshop (using bicubic sampling) and then shrink them back down again during compression. Thankfully, I was able to automate it as part of the build process.