Linking to other sites: just type the web address. E.g., http://grault.net/adjunct/ becomes http://grault.net/adjunct/
Using images: just type the web address. E.g., http://grault.net/adjunct/adjunct.png becomes
At the bottom of the page when editing you'll see a box labelled "Summary". Type a brief description of the way you changed the page. If you made an unimportant change, such as fixing a spelling mistake, enable the "This change is a minor edit" checkbox below it. That will cause your change to be hidden in the default RecentChanges view. See RecentChangesOptions for more on this.
Note that <tab><space>:<tab> for block indents does not migrate seamlessly. You'll need to change it to :<space> to prevent it from becoming monospaced.
Older versions of pages are kept around for a while. Readfor more details.