There is a subtle difference between html and xhtml options, some tags aren't closed in html option, for example. html is used by default, but I would always use xhtml, html seems obsolete to me.
It usually doesn't matter if you put your configurations before or after \begin{document}. In some cases you may get error missing \begin{document}, in this case you should put the configuration which causes this issue after begin{document}. I must admit that I don't recall which configuration can cause that, but I am sure that I approached this issue.
Each \Configure graphics may take different number of arguments, it depends on how it was declared. \Configure{graphics*} takes two arguments, the first one is included format, which is configured, the second is code which should be executed to include that image. It usually contains code to make conversion between different image formats, command to include the resulting image in the list of files used by tex4ht and finally command to include the final picture in the document. Some documentation can be found here.
Command
\Needs{"convert \csname Gin@base\endcsname.pdf
\csname Gin@base\endcsname.png"}
declares that command convert should be executed. \csname Gin@base\endcsname contains the image name. As this command contains @ characters in it's name, you can either enable it using \makeatletter and the you can just use \Gin@base, or you can use this \Csname ... \endcsname construct. In this case you don't need to use \makeatletter.
\Picture[pict]{\csname Gin@base\endcsname.png}%
is used to include the image in the document and
\special{t4ht+@File: \csname Gin@base\endcsname.png}
declares the image file as used. It is important if use different output directory for your document, the image wouldn't be copied if you don't use this construct.
Regarding config file place, you can put it in you local TEXMF tree, somewhere in the /tex/latex directory, it would be something like /home/michal/texmt/tex/latex/mytex4htconfigs/mycongig.cfg on my system.
{}on the gui. – Oct 17 '16 at 21:24\Preamblecommands you ask about are for producing xhtml and html, respectively. These are used when you run your document through htlatex. TeX4ht is complicated and not particularly well documented. A good starting point to learn how to use it is the tutorial. – Oct 17 '16 at 21:31/usr/local/texlive/local. For more info ,see for example, http://tex.stackexchange.com/questions/1137/where-do-i-place-my-own-sty-or-cls-files-to-make-them-available-to-all-my-te – Oct 18 '16 at 02:24