The simplest approach would be to just use an image editing application and save it as an image.
Thanks, but then the text isn't searchable. Besides which, I may have other uses for floating objects, assuming that I can get them to behave.
That's fine and I do understand it. As I said, the approach I offered was only the simplest. Not the only.
Please note that RoboHelp isn't really set up to easily allow you to create this situation in the manner you are wanting. So be prepared for a bit of a struggle and gnashing of teeth just to get the text to be searchable. You will have to step outside the "comfort zone" of typical editing of RoboHelp topics and manipulate things by changing the code and the CSS by hand. Don't be too surprised to find that when you do this, RoboHelp will often "helpfully" re-write the code in many cases and make you jump through hoops to keep it working.
It's your circus to run and you are totally free to have as many monkeys as you like!