UIImage couldn't load some png picture

I Use the following code to load png image:

UIImage *imageBack1 = [UIImage imageNamed:@"Bar1.png"];
UI开发者_如何转开发Image *imageBack2 = [UIImage imageNamed:@"Bar2.png"];

imageBack1 work right when imageBack2's value is nil, Bar1.png and Bar2.png are located at the same place,but why Bar2.png couldn't be load?

In fact , the problem come from the png file,but not cocoa. Iphoto couldn't recognize it.