Hiya Paul,
Although it's not the most widely used way, did you try using inline styling? Usually that will override whatever is included in the CSS files. I know many people have added a lot of different images and styles of images to their sites so I'm sure it can be done. Coding in the SCC programs is a bit strange though I'll admit. Many times you have to put the tags themselves within the HTML boxes [[[ ]]] and the content outside of it. Something like just centering a line of text or image would be like this:
[[[<center>]]]Text here you want to center, style this text using the tools above in the program, not with CSS[[[</center>]]]
With that you have the ability to then apply any text styling to the text via the program itself rather than using CSS. Same for an image only you'd put that within the boxes like so:
[[[<center><img src="http://imagenamehere.png" alt="Image Alt Here"></center>]]]
I'm sure you can take this a step farther and use inline CSS styling like so:
[[[<center><img style="border: 0; whatever other image styling you want;" src="http://imagenamehere.png" alt="Image Alt Here"></center>]]]
You don't need the CSS for text though, that you do within the program, just the centering of it is done through tags as shown above.
Hope that helps some.