The default behavior is to have them in a drop-down format, with the images being changed to images of a simple product using Javascript. If you want to change this functionality, you will need to change the assistant that creates these parameters.
I do not have a clean installation on mine, and I have a module that completely overwrites part of the parameters, but you will need to find where the configurable product information displays these drop-down fields and changes them to the radio with a simple product image. I'm sorry that I have no special features for you, but this should get you started.
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