I’ve noticed the same problem.
As far as I can see the path has changed with release 0.9.5 of ModX, due to the new feature ’Manager theme’ in the configuration settings.
The images are now found in /manager/media/style/’MODx-theme’/images and /manager/media/style/’MODx-theme’/images/tree, where ’MODx-theme’ can be MODx, MODxGreen or MODxLight (depending on your settings).