Color Quantizer - the best bloody well kept secret on the web for PNG optimization. I am not kidding here: if you care about download size, optimizing those PNGs individually after exporting from Construct will save you at least 30%~50% (depending on the content).
Here are some examples from our game. (The originals on the left were exported with brute settings in Construct)
This is an non-lossy exported png file, thus making it viable to compare both versions.
One of the reasons Color Quantizer is brilliant in optimizing png files is that you have full control over the number of colours, and that a quality mask brush tool offers you the option to perfectly optimize smaller areas which may be reduced in quality due to automatic conversion. CQ gives you absolute manual quality control over the smallest details in the final version.
For example, the chimney for the small building asset had some pronounced issues with the initial automatic conversion - I then used the quality mask brush to paint over the affected area, and clicked OK once more to recalculate the optimized version. Problem solved: far less colours, but still (visually) the exact same quality.
The windshaft in the center of the sails had anti-aliasing issues, which I again solved with the quality mask brush tool.
It is beyond me why so few people are aware of CQ. I mean, look at those numbers for these four assets:
246.2kb --> 33.6 kb!!! Seven times smaller!
Granted, not every asset may benefit as much as these examples, but in overall CQ results in savings at 50% or more. And no difference in visual quality.
Color Quantizer is THE definite PNG optimizer. Nothing else comes close. I cannot live without anymore. And it's free!
Get it here: http://x128.ho.ua/color-quantizer.html