To modify your POSReady 7 image just insert the POSReady 7 DVD containing the Distribution Share (DS) and launch DISMUI.
The tool will show you immediately which features are currently installed on the system.
If you uncheck a feature it will be removed from the system.
Please note that Microsoft only supports adding features to the system when it is offline.
This means that you should boot from the POSReady 7 setup DVD and add features from there while the image is not running.
You can also use DISMUI there however since DISMUI needs the .NET Framework to run you will need to do additional preparation work once.
I have already described how to make DISMUI portable using the open source implementation of .NET called Mono. Here is an adapted guide especially for POSReady 7:
You can now use the USB key containing DISMUI and Mono to run the tool within the POSReady 7 setup environment.
For this boot you POSReady 7 setup DVD and press Shift+F10 at any time.
Navigate to your USB key and launch the previously created batchfile or shortcut.
Within DISMUI select the “Offline” option and enter the path to your POSReady 7 image.
Now you can add features easily by just following the wizard.