Hmmmmm..... seems to me it WAS in the docs - lol... I just added the following, Adolfo,
DragMath is a GUI constructor that allows you to create text expressions from a GUI representation of your equation. IT DOES NOT display your equation. To do that you need a display applicaton installed. While Moodle comes with an antiquated but functional Tex filter, that filter DOE NOT include Tex itself (though it does include a fallback to mimetex, another application discussed in Mathematics. There are also other display applications discussed there, such as ASCIIMathML and MathJax. As noted below, DragMath can render an equation in a variety of formats each with its own tokens, or can insert the text expression with no tokens at all.
The references you mention to ghostscript or imagemagick have to do with the use of the Moodle tex filter, which relies on two Tex binaries and convert, anorther binary which can be obtained from the GS or IM distriubtions (though there is a hack to buy pass the ned for that binary, convert.)
If you go to Moodle docs on Tex you will find a good deal there; focus on using texdebug and determine if the requisite binaries are already on your system. You may want instead to install MathJax or ASCIIMathML. Both of these install rather quiickly and though they require a quick hack to a file, are not otherwise internal to Moodle (no filter)
ASCIMathML is just about the quickest and easiest way to get math notatin up on a moodle and it can support SVG as well. I have proposed including ASCIIMathML in core moodle because it provides a completey self-contained solution while the current filter really does not (whle mimetex is included, changing perms on it is one of the most frequent issues presented here, while asciimath is simply a js file.) I am also impressed with using MathJax with jsxgraph. While either of these is not difficult to manage, running the both together requires a little finesse.