You can customize and add theme to Droppics, but we don't use the standard MVC Overrides but the Joomla default plugin system.
Droppics themes are in fact plugins and they are located in the folder /plugins/droppics
You'll have to copy one plugin, change the name of the plugin to create your new plugin (pluginname.xml and the class name of the plugin). Then install it through Joomla standard installer, it should be shown in the list of themes. You can then do all changes you want in this theme, it will not be affected by updates.
Droppics is built to be used in every WYSIYG editor field, you just need to have content plugin to be called. Most of the time it's activated by default.
It's not the case in the Joomla HTML custom module, you need to activate it.
In the option tab you need to set the option Prepare content to Yes.
Yes, because Droppics is keeping a copy original images in order to regenerate thumbnails.
So when you change image size in parameters you'll be asked to regenerate thumbnails.
If you don't regenerate thumbnail old size will be used for galleries already online.
Droppics is stored your images under: /images/com_droppics/ID-of-the-gallery.
So you can access to all images with other image tool if you want.