Change iPhone UISlider bar image

You were right to use -setMinimumTrackImage:forState: and -setMaximumTrackImage:forState: methods. What you missed is that you should provide stretchable UIImage to them, the rest is taken care of automagically:

UIImage *sliderLeftTrackImage = [[UIImage imageNamed: @"SliderMin.png"] stretchableImageWithLeftCapWidth: 9 topCapHeight: 0];
UIImage *sliderRightTrackImage = [[UIImage imageNamed: @"SliderMax.png"] stretchableImageWithLeftCapWidth: 9 topCapHeight: 0];
[mySlider setMinimumTrackImage: sliderLeftTrackImage forState: UIControlStateNormal];
[mySlider setMaximumTrackImage: sliderRightTrackImage forState: UIControlStateNormal];

You can use the same image for both the min and max parts.

The simplest solution would be to render a UIImage behind the control to represent the track. The thumbknob can be changed with:

[mySlider setThumbImage:[UIImage imageNamed:@"thumb_image.png"] forState:UIControlStateNormal];

A side-effect of this is that the track isn't rendered unless you provide your own. This, combined with the UIImage, provide you with a custom UISlider without you having to subclass anything.