Allow me to show you what pngcrush can and cannot do by illustrating it using Wikipe-tan, the moe anthropomorphism of Wikipedia (click on the images to see them at full size).
Here is the original Wikipe-tan image (well, actually a PNG rendering of the original SVG vector graphics):
Applied to this particular image, pngcrush can generate a file that is 20 percent smaller – without any loss of image information.
|Wikipe-tan after pngcrush|
While pngcrush reduces the image size considerably, it cannot free you from optimizing the image, for example by deciding whether reducing the amount of data in a lossy manner is acceptable.
|Wikipe-tan, 256 colors |
For the last image it made no difference whether I use pngcrush or not, GIMP already created the optimally compressed version that is 72% smaller than the original. Whether the loss in quality is acceptable for this image or not depends on the background; on an unstructured background it may not be the best choice but if the background has some structure it works quite well. As always: It pays to know what you are doing ☺
As far as running pngcrush there is not much to say. Here is a normal run of the program:
And here is a run with the -brute option that tries each and any possible way of compressing.
Note that while it sometimes yields better results, using -brute usually is just waste of time.