The PNG compression isn't lossy, the grades of compression are how long it takes to make them, so that shouldn't matter.
Usually it's when the PNG is smaller than the size it's displaying at run time. If the screen stretches to fit a bigger screen size, and the PNGs weren't bigger than needed, they will blur to cover the scaling.
(In other words, if you make the PNG the biggest size it could stretch to, and then size it down at design time, it will have room to grow and stay sharp.)
However that may not be your problem, that's why the capx suggestion, so we can see what's happening.