1: The part you change is the part in blue bold. It is to be changed to the server path for your forum directory, not an HTTP link, and it is easy enough to determine if you do not know it:
Code:
Content visible to registered users only.
2:The pro version has a lot more
features than the lite version. There is a lite demo
here and a pro demo
here.
3: The lite version is good if you expect your gallery to remain small, and while I haven't done any benchmark testing, I would guess that pro is less server intensive than lite as pro allows for more static image links and has had its queries optimized. You can find page generation time and the number of queries run in the footer of the pages on this site. These values are representative of this site only, as your site, server, and/or number of modifications may be different, but they should at least give you an idea as to intensity in relation to this site.