No, it doesn’t involve any advanced programming.. The template chunk can be pure html with the available placeholders included. The default content may seem complex, because it is able to handle all the various display types that are controlled by snippet parameters (display and embedtype).
Here’s example of minimum template contents:
&galleryOuterTpl
<div>
[+maxigallery.pictures+]
</div>
&galleryPictureTpl
<a href="[+maxigallery.picture_link_url+]">
<img src="[(base_url)][+maxigallery.path_to_gal+]tn_[+maxigallery.picture.filename+]" title="[+maxigallery.picture.title:htmlent+] [+maxigallery.strings.click_to_zoom+]" alt="[+maxigallery.picture.title:htmlent+] [+maxigallery.strings.click_to_zoom+]" />
</a>
You can see the available template placeholders from MaxiGallery wiki page:
http://wiki.modxcms.com/index.php/MaxiGallery#Placeholders
If you would like to use slimbox or something, you would have rel="lightbox[set]" etc attributes in the link.. You can see examples from the default template content.. There is a block like this for example:
[+maxigallery.embedtype:is=`slimbox`:then=`
<a href="[(base_url)][+maxigallery.path_to_gal+][+maxigallery.picture.filename+]" rel="lightbox[maxigallery]" title="[+maxigallery.picture.title:htmlent+] - [+maxigallery.picture.descr:htmlent+]">
`+]
What means that if the snippet call has &embedtype=`slimbox` this link is used. But you can cut most of this stuff out from your custom template because you only need the code that you will use for your display type.