Is it possible to add a watermark just for pictures in new galleries(since a special date)? In the old ones I've added the watermark manually in the pictures, so the zenPhoto generated and the manually generated would overlap.

It is possible, but would take some work. (I have not tried this, so it may not be 100% accurate.)

You would need to have the global watermarking options turned off (no watermark). Then you would need to create a plugin that attached to the new_image filter. That filter would call the cacheImage function with the appropriate parameters to cause it to create the cached image including a watermark.

Of course, if you do this and purge your cache, all those watermarks will go away.

There's a much easier way than that. Turn global watermarking off, then set watermarks per album. It's in the album administration. You'd just have to remember to set it to on for any new albums you created.