There is more than one way this can be done.
One way is to create a custom Spark Button skin, and then use the BitmapImage component, along with the "up" state.
You specify the new skin for the Spark Button using the skinClass property, for example:
In the custom skin, you can use BitmapImage to embed your image for the up state, as follows:
You may also want to use height/width and/or constraints in order to position the image as desired on the Button