I'm not sure of your exact scenario, but you should be able to use CSS inside of an HTML wiki just as you could on any other HTML page.

However, if you're getting your R images via a call to LABKEY.WebPart, I can imagine it getting more complicated to control the layout as you desire.

You may be able to put your desired CSS on an enclosing element and tell the web part to render into it. Or, you might need to use the success callback to manipulate the DOM that is returned to apply the desired CSS.

If that isn't helpful or relevant, if you have a code snippet of what you're doing right now, that will help guide additional suggestions.

Thanks,
Josh