First subclass: the NSView
---------------------------------
#import "customViewer.h"

@implementation customViewer

-(void)awakeFromNib
{
    image = [NSImage imageNamed:@"bgImageOne"];
    [self setNeedsDisplay:YES];
	
}
-(void)drawRect:(NSRect)rect
{
    [[NSColor clearColor] set];
    NSRectFill([self frame]);
	[image compositeToPoint:NSZeroPoint operation:NSCompositeSourceOver];
}
- (void)mouseDown:(NSEvent *)theEvent
{
	BOOL isDoubleClick = [theEvent clickCount] == 2;
	if(isDoubleClick == YES) 
	{
		image = [NSImage imageNamed:@"bgImage_over"];
		[self setNeedsDisplay:YES];
	}
}
-(void)mouseUp:(NSEvent *)theEvent
{
	image = [NSImage imageNamed:@"bgImageOne"];
	[self setNeedsDisplay:YES];
}
-(void)setNewImage         // THIS is the method I want to call from the other class.
{	
	NSLog(@"image method called");
}

@end

The NSTextField subclass:

#import "customTextField.h"


@implementation KGSerialFieldLabel
-(void)mouseDown:(NSEvent *)event
{
	BOOL isDoubleClick = [event clickCount] == 2;
	if(isDoubleClick) 
	{
	[customViewOutlet setNewImage];   // The message that should be set to customView (customViewOutlet is an IBOutlet connected to customViewer)
}    
}
@end