Vanilla Distribution Image Optimization
I recommend that Vanilla distribute all images in the release package after running them through the above utilities to reduce their size, thus reducing loading times. (Images, unlike html, do not get sent through gzip compression when served by servers.)
There are several unix command line utilities to almost-losslessly reduce the size of images through compression and other optimization techniques.
For OS X there is a stand alone application gui to the above utilities that does the same thing: http://pornel.net/imageoptim/en
Running them on the default images provided with Vanilla shows that a significant file size savings can be achieved (with no image quality degradation).
Here is a screen shot with the compression achieved and the new image file sizes.