RetroFE doesn't have a lot of fixed art types. Other than video (expected in the video directory), and a few reserved words, a theme creator is completely free to determine the names of the art types. The examples mentioned in the settings.conf files are the names used by the default themes for RetroFE, but if you want to use a directory called "woopsie", you could just use type="woopsie" in the layout.xml of your theme.
RetroFE supports a number of modes for artwork to determine if the art should be part of the collection (to be used by many themes) or part of the theme itself. In addition, it supports modes for artwork that's specific for a collection and artwork that's common for all collections (e.g. manufacturer logos).
You can find the possible file locations of types in
http://www.retrofe.nl/wiki/doku.php?id=layouts#mode_attributeBasically, you need to figure out what types are used by a theme to know where your files should reside. Luckily, most themes use the standard types defined in the settings.conf file (artwork_front, artwork_back, medium_front, medium_back, story, logo, screenshot, screentitle, and video if I'm not mistaken). Not all themes use all art types though.
Hope this helps a little; you should get the hang of it pretty quickly.