No. In creating a new style I created an entirely new style, then created a child of that which inherits most of the default styles. According to the manual it shouldn't be possible to change the default styles by doing this and, even if it were, there's a 'restore default/previous' for each and every option.
I did rename the files a couple of times though becuase my desktop has filled up with variations on 'image.xxx'. I also reduced the image bit count from 32 to 4 but I'm unsure if the latter would have an effect on rendering.
edit:
>>fixified<<
Oh, yes!
They're still transparent, too, which works with my greeny style, though I should probably create adjusted images that blend in with that style....