Construct 2 converts everything to 24bit PNG with alpha channel when it's imported into C2.
I always just save everything as 24bit PNG with alpha so it doesn't have to do any conversion... and also so there's no compression on the original images.
Try to make your graphics at the "native" size of your game/app so that you aren't having to resize them larger or smaller in the game itself (unless you're adding a zoom feature that would scale the entire layout instead).
Also, make sure that if you're doing retro-style artwork (no anti-aliased edges, etc) that you set your resize sampling to POINT, otherwise leave it as LINEAR if you're using "soft" or anti-aliased artwork.
~Sol