I use the code below to get the width and height of a NSImage:
NSImage *image = [[[NSImage alloc] initWithContentsOfFile:[NSString stringWithFormat:s]] autorelea
Dimensions of your image in pixels is stored in NSImageRep of your image. If your file contains only one image, it will be like this:
NSImageRep *rep = [[image representations] objectAtIndex:0];
NSSize imageSize = NSMakeSize(rep.pixelsWide, rep.pixelsHigh);
where image is your NSImage and imageSize is your image size in pixels.