I've been using Paint.NET on-off for several years now: I recall asking here or somewhere once about how to use optipng with the PNG output. But tonight I learned about an SIMD library (YEPP) that Georgia Tech worked on. Not sure how much it'd compete with/compliment the "multicore JIT" mentioned on the blog, but it would definitely appear useful for imaging software.
* http://www.yeppp.info/
* http://blog.getpaint.net/2012/10/19/more-on-multicore-jit/ * http://en.wikipedia.org/wiki/SIMD