First. Thank You for the great free software. I've seen this 3 days ago and I already replaced my copermine with ZenPhoto.

Since I have no experience with Apache I have a maybe stupid question.

Looking this example:

!!! Change this to the web path (eg: http://www.example.com/photos --> /photos)

RewriteBase /zenphoto

I'm a little stun because my gallery is in a subdomain gallery.ForexSB.com

so what to put after RewriteBase? or to delete it from the .htaccess
after some attempts I make this:
RewriteBase /

and it seems it works. Is to OK?
Thanks!!

What you did is correct. The base is for if you have your gallery in a folder within your site. If it is at the root of the site as you have done, then the base is the root of the site like you have made it.