You should put one of those images in a Capx demonstrating what is happening. That way we can see what you mean. (if it's your graphics drivers, we could tell you we don't see the problem.)
Is the image clear in constructs image editor? If it's clear there, then your formats have nothing to do with it.
I'm fairly sure construct uses all png-24, as that's the one that has transparency alphas. If you don't get an option to output the type, it's probably saving using whats there, if it was a gif, it will use -8, if it has too many colors or transparency then -24.