rgdot's reply really worked for me.
The Images folder goes in the same "Miscellaneous" folder as StandaloneStack2.exe (C:\Program Files\Circle Dock\System\Miscellaneous\Images). Since my Stack folder was all shortcut links, I named the PNG files the same as the documents or programs, and voila, pretty, bright, clear icons in my stack grid. I did try the ".[extension].png" method of showing the PNG files too, and that worked, but as I said, all of my stack were *.lnk so that wasn't useful for me.