Why not something like this:
You will need an outlet to an NSImageView.
IBOutlet NSImageView *imageView;
When the user clicks the button, it will call an action. This action will have something like:
-(void)actionName {
...
[imageView imageNamed:[NSImage imageNamed:@"name.gif"]];
...
}
Make sure that gif is added as part of your project!
Here are the NSImageView and NSImage developer pages:
NSImageView
NSImage