If you make the rebranded image the same name as the old image and place it in each project folder in the same location, that is all you'll have to do.
That's OK if the images were inserted originally in Rh9 or 10. If they were inserted in an earlier version or if they were resized in 9 or 10, then they will have size settings applied so even a small variation in size could give some distortion. Something you need to check.
Like Colum, I am assuming the file type is unchanged.
See www.grainge.org for RoboHelp and Authoring tips
Thanks very much for your helpful replies - much appreciated. The image in question was a different size and file type, and was added in RoboHelp 5, so I decided to replace it manually.