The best way I found to deal with this is to paint the background color of your Delphi toolbar (or whatever UI element) underneath every pixel that contains transparency in Photoshop before saving the PNG image. This way, the toolbar icons are totally opaque except for the 100% transparent pixels, so there is no alpha blending that takes place--and hence no ugliness.
Use RRUZ's code here to make the disabled toolbar icons look better.