I can't help you with the banners as it is functionality I don't use myself. However, managing the file galleries is quite easy. Do you not see a link create new gallery when you click on the 'File Galleries' menu? Make sure you have the right permissions to do so.
The RSS feeds can be a bit of a problem. First of all there is no proxy support and Tiki does have problems with some type of RSS feeds. I think several bugs are already logged for this on the SF site.
Most of the Nukes are indeed faster than Tiki but also much more difficult to manage and far less consistent then Tiki where functionality is concerned. I use PostNuke for a while but would never go back to it after getting to know TikiWiki!