We shall export a skin, for working on it in Photoshop, and import it again in Sketchup.

I make a rectange in SketchUp, and export it: File > Export > 3D Model > 'Collada File (*.dae)'. I get, apart from the DAE file, a folder with a 2D skin, a JPG, which I open in Photoshop.

I then work on that and export it as a JPG again.

In SketchUp now proceed as follows. See below.
Be sure that A is at 'In Model'. Click B so that C appears.
Click D (Create Material). Second panel E appears. At F find the skin JPG. Click OK. G appears.

You now have the skin uploaded in SketchUp, see below.

You can use it as any skin, but mind you: you have the skin In Model, in the file that is open now. Close that file without saving and you loose the skin. You can right click the imported skin and save it as an SKM file, SKetchup Material. Save it at Program Files > SketchUp > SketchUp [versie] > Materials, in a new folder. Restart SketchUp and the new folder will appear among the already existing, at H.

As from Photoshop 2015 you cannot import SketchUp objects covered with more then one skin (material).