You'll be needing to become familiar with the Material Room.  From there you can see the exact address of each texture that is applied to your figure.

Else, you can DL the Free P3DO Explorer(like Windows Explorer) and lick on the .CR2 you're thinking of.

Either way once you've located the texture, you'd do best to create a folder, named "Originals," in that same directory, and put a copy of the texture you're interested with in there.  This way you can just open and save over the texture where it is, without needing to change it's location in Poser.  Then if thins go haywire, you can just drag a copy of that Original into it's above directory, and overwrite.

Another note, once you save your new texture, you'll need make sure Render > Reload Texture > Keep Textures Loaded is'n checked.  Then you can click Render > Reload Textures, and the changes you made in Photoshop will show up in Poser, on the fly(Without needing to restart either application.

BTW - It' easier to use Reload Textures from the Render Panel vs. the Document Window.  At the document window you'll then need to wait while the textures are immediately loaded.  From the Render Window, you can then use the Plop Render/Render and the changes will load during the render.