Some PNG images although just 150px x 160px wide have 60KB filesize. I've already run them though optimizers (ImageOptim for Mac) but that didn't help much. 

Is there any way to compress it further, either manually or using some other tool? 60KB for a normal 150x160 image is really unacceptable and I can't use JPG or GIF there because I need transparency.

I tried exporting the PNGs as **8-bit PNG** and the filesize has reduced significantly!