According to this answer there are several .net libraries that can be used to optimize the png size
The tools I use for png optimization are:
- Convert to png8: If there are few colors (say, screenshots) then I use [pngnq][2] or Gimp's Indexed color mode to quantize down to 256 colors. Png8 can be smaller than png24 or png32. For details see [PNG8 – The Clear Winner][3].
- Optipng, a fast general png optimizer. [C# PNG Optimization Tutorial][9] has details on how to run optipng from C#.
- Finally [pngout][4] is slow but often (80-90% of the time) manages to squeeze the png down further than optipng. Run optipng first though, as optipng will automatically do other optimizations that pngout does not attempt.