// Set our height and width (using ebonics)
float wiff = 10.0;
float heiff = 10.0;

// Create a new image for us that will will draw the other images on to.
NSImage *finalImage = 	[[NSImage alloc] initWithSize:NSMakeSize(wiff, heiff)];

// Grab our overlay and mainImage from our file system.
NSImage *overlay 	= 	[[NSImage alloc] initWithData:[NSData dataWithContentsOfFile:@"/path/to/overlay_image.jpg"]];
NSImage *mainImage	=	[[NSImage alloc] initWithData:[NSData dataWithContentsOfFile:@"/path/to/main_image.jpg"]];

// Lock focus, this way we can manipulate the image we are focused on
[finalImage lockFocus];

// First we draw our mainImage on the image we created
[mainImage drawInRect:NSMakeRect(0, 0, wiff, heiff) 
			  fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:1.0];

// Now let's draw our overlay image on top of our mainImage with a bottom/left offset of 10
[overlay drawInRect:NSMakeRect(10, 10, [overlay size].width, [overlay size].height) 
		 fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:1.0];

// Finished drawing, let's unlock focus.
[finalImage unlockFocus];

// Create a data object from the image we created.
NSData *finalData = [finalImage TIFFRepresentation];

// Now we write our image to file and then we can go
// to the track and place a cool G on the horse "Cocoa gives him runs"
[[[NSBitmapImageRep imageRepWithData:finalData] 
  representationUsingType:NSJPEGFileType properties:nil] 
 writeToFile:[NSString stringWithFormat:@"/path/to/folder/new_image.jpg"] atomically:YES];