Here are two custom components that extend the CheckBox and RadioButton classes to render an image in between the checkbox/radiobutton icon and label.
The main property to set is imageSource which acts just like the Image.source property.
The image position depends on the labelPlacement property (left, right, top, bottom) and the horizontalGap style.
You can also set the imageWidth and imageHeight properties to scale the image down. If you only set one of those properties then the other one is calculated to keep the image's width and height ratio the same.
Note that the ImageCheckBox actually extends my flex.utils.ui.CheckBox class and not the expected mx.controls.CheckBox class.
The flex.utils.ui.CheckBox class supports backgroundColor and backgroundAlpha styles, and as a result the background is always painted. But the default alpha value is 0, so usually you won't see the background unless you set the background color and alpha. The reason for this is that the default mx.controls.CheckBox class has a gap between the checkbox icon and the label (which is the horizontalGap style). This gap causes mouse out events to fire when your cursor moves from the icon or label into this gap. As a result the checkbox tooltip will disappear, and the checkbox will not have the rollover styles. So by painting an invisible background color over the whole area of the checkbox this problem doesn't occur. Same thing applies with the RadioButton class.