Customizing Basic Icons

I just want to make sure I'm not missing something.....

I type in "Custom Icons" and I get instructions that take me to the basic, preassigned icons available.  

But I want my own icons....I would like to add icons to the basic (already loaded) icons available.  For instance, there is no icon for a tent or freshwater location.  Or change the colors of the icons.  These are basic features I've been using on other platforms (but Gaia has a better map and mapping system).  

Am I missing how to do this or can it not be done?