tabbar item image and selectedImage

Question

I have a tab bar controller (its a tab bar based application, so tab bar is on MainWindow.xib). In this xib, I have added 4 tab bar items and I have set the image of all tab bar item. Due to this, I am facing 2 issues:

1) The image is white-colored, but when I run the application, its showing all the images on tab bar item as gray colored. How can I make it look same as is in the original image.

2) I have a selected image, that I want to add on the tab bar item which is currently selected. How should I do this???

UPDATED AFTER NICK's CODE:

Hey, in iOS 5, you will have to write following code in your app delegate for setting tab bar item selected and unselected image (the category solution will work only on 4):

if ([[[UIDevice currentDevice] systemVersion] floatValue]>4.9) {
    NSString *selectedImageName,*unselectedImageName;

    for (int counter = 0; counter < [self.tabBarController.tabBar.items count]; counter++) {
        if (counter==0) {
            selectedImageName = <someImagename>;
            unselectedImageName = <someImagename>;
        }
        else if (counter==1) {
            selectedImageName = <someImagename>;
            unselectedImageName = <someImagename>;
        }
        .
                    .
        else {
            selectedImageName = <someImagename>;
            unselectedImageName = <someImagename>;
        }
        UIImage *selectedImage = [UIImage imageNamed:selectedImageName];
        UIImage *unselectedImage = [UIImage imageNamed:unselectedImageName];

        UITabBarItem *item = [self.tabBarController.tabBar.items objectAtIndex:counter];
        if ([item respondsToSelector:@selector(setFinishedSelectedImage:withFinishedUnselectedImage:)]) {
            [item setFinishedSelectedImage:selectedImage withFinishedUnselectedImage:unselectedImage];
        }
    }
}