I just posted this on the other thread on psds but thought it might fit well here....You really don't need psd files to modify these themes, they're purposely very simple in construction.
If you want to modify a theme using images it's very simple in photoshop. Just get the image from your image pack and open it up. Create a new layer and build your image directly on top of the current one. When you're done, hide the layer containing the old image and export it using the same name and file type as the old one. Upload it to your server and it will fit exactly as the old one did with no modification even of the CSS.