Thanks for that.
It worked but with some drawbacks. The original image I was using was an Arroway texture so it was tiled. Painting a rectangle with the edited version and then saving it and reimporting loses the tiling. Also, for some reason the original wrapped itself around the tree trunk cleanly, showing little of the faceting that occurs when wrapping a cylinder with a texture but the re-imported image breaks up along the facets. Not sure why.
I guess the answer is to go back to the original image, save it under a new name and make the necessary changes in Photoshop. But I find that to be unfortunate because, minor though it may be, SketchUp's material editor seems to always give me exactly what I want, whereas Photoshop tends to feel like taking a battleship out for a Sunday boating on the lake.
Anyways, thanks for your reply, I'm sure the technique will prove useful in many situations. Have a good Sunday ...