This should work, assuming you don't want any image transparency and composition effects.
-(void)drawRect:(NSRect)dirtyRect
{
[super drawRect:dirtyRect];
NSImage *anotherImage = [NSImage imageNamed:@"alert.png"];
[anotherImage drawAtPoint:NSMakePoint(10,0) fromRect:rectToDrawImage operation:NSCompositeCopy fraction:1.0];
}