UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"图片"]];
/*
UIViewContentModeScaleToFill,
UIViewContentModeScaleAspectFit, // contents scaled to fit with fixed aspect. remainder is transparent
UIViewContentModeScaleAspectFill,
*/
imgView.contentMode = UIViewContentModeScaleAspectFit;
Fill(填充)，Fit(试用)，Aspect(方向)，Scale(比例)
