Plank-Reloaded uses the icons of the application. If you want to have specific icons for plank then you will need to create a desktop file with a path for the icon you wish to specify. If your talking about changing the system icons you will need to do that through your settings. Someone who use’s KDE can tell you where to change icons at in KDE. this will reflect in your plank. If its the latter an you wish for new icons start here