I'm an experienced developer but a relative newbie with Cocoa. The Cocoa documentation says that toolbar items (NSToolbarItem) are 32x32 in regular size and 24x24 in small size. It goes on to say that you can (and should) include an image that includes both sizes and that a multi-page TIFF or an ICNS file can be used. This is where I am confused. I have tried several different programs that create ICNS files but all of them create files including 16x16, 32x32, and bigger. Is the assumption that the 24x24 toolbar icon uses a 16x16 image or is there a way to create a ICNS file with a 24x24 image? Or are people using an application to create multi-page TIFF files?
Use Icon Composer.app and supply the 16 and 32 sizes. The system will automatically up- or down-scale between them as needed and usually does a very good job of it.