UIBarButtonItem *doneitem=[[[UIBarButtonItem alloc]initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:self action:@selector(donePressed:)]autorelease];
Please try this code :
UIButton *btnNext1 =[[UIButton alloc] init];
[btnNext1 setBackgroundImage:[UIImage imageNamed:@"btnNext.png"] forState:UIControlStateNormal];
btnNext1.frame = CGRectMake(100, 100, 50, 30);
UIBarButtonItem *btnNext =[[UIBarButtonItem alloc] initWithCustomView:btnNext1];
[btnNext1 addTarget:self action:@selector(nextButtonClicked) forControlEvents:UIControlEventTouchUpInside];
self.navigationItem.rightBarButtonItem = btnNext;