Вот как я юзаю BFImage.

gistfile1.txt

NSImage *backgroundImage = [[NSImage imageNamed:@"PopoverBackgroud.png"] stretchableImageWithEdgeInsets:[self backgroundImageInsets]];
NSImageView *backgroundImageView = [[NSImageView alloc] initWithFrame:contentRect];
backgroundImageView.autoresizingMask = NSViewWidthSizable | NSViewHeightSizable;
backgroundImageView.imageScaling = NSImageScaleAxesIndependently;
backgroundImageView.image = backgroundImage;
[_positioningWindow.contentView addSubview:backgroundImageView];
[backgroundImageView release];