I wouldn't overwrite the images in those directories though.
Create a new directory /ubbthreads/images/general/custom, copy and paste the existing images that you are using in to that and then change them to whatever you want. That way, if you decide to revert back to default images you can just change the preferred folder, rather than having to upload the default images again.
Once you've done that you can select the folder containing those images in your styles area.
You can do this for any of the directories:
/ubbthreads/images/graemlins/custom
/ubbthreads/images/icons/custom
/ubbthreads/images/moods/custom
etc.
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