Construct's built-in PNG compression is very good, but is lossless, so will preserve artwork pixel-perfectly. Using other PNG compression tools which are lossy can improve the compression, but these fundamentally work by degrading the quality of the artwork in your game. So you may want to check the appearance of your game very carefully afterwards to make sure it hasn't degraded the quality too much. For example a nice smooth gradient could be much smaller if the colors are reduced, but then it can appear with visible bands instead of a smooth gradient.