Buttons have a number of different states, and you can set the button image once for all states or assign different values for each state.

To set the ‘default’ image for a button, call SetImage for UIControlState.Normal and that image will be used for all states:

buttonRect = UIButton.FromType(UIButtonType.System);
buttonRect.SetImage(UIImage.FromFile ("star-gold45.png"), UIControlState.Normal);

Alternatively you can make each state different, as this image shows:

For most button types, only two states – Normal and Disabled – need to be set (the blue highlighted color is automatically applied to the transparent parts of the image):

For the Custom button type you must supply a separate image for the Highlighted state if you wish the button to look different when pressed.

buttonStarCustom = UIButton.FromType(UIButtonType.Custom);
buttonStarCustom.SetImage (UIImage.FromFile ("star-gold45.png"), UIControlState.Normal);
buttonStarCustom.SetImage (UIImage.FromFile ("star-gold45_sel.png"), UIControlState.Highlighted);
buttonStarCustom.SetImage (UIImage.FromFile ("star-grey45.png"), UIControlState.Disabled);

You can also set the different states using Interface Builder by choosing each one from the drop-down-list in the Attribute Inspector (see the Set Button Text recipe for an example using Interface Builder).