Modding Basics

You can create mods from in game. The editor allows you to choose from four things to change. First you can change the base game, this is not recommended unless you want to make sure that it will take effect in every game.

The next part of the editor allows you to create mods, which covers almost everything that fills a world, thirdly there's the scenario editor, which includes the map editor and also allows you to place POI's and populate the world. The last option is the Assets editor which allows you to put custom assets in the game and which is also used to connect assets to races, Old Ones etc.

Mods
The Mod editor allows the creation of the following elements:
 * Races
 * Cultures
 * Governments
 * Old Ones
 * Agents
 * Traits
 * Abilities
 * Tags
 * Skills
 * Classes
 * Spells
 * Challenges
 * Events
 * Items
 * Lore
 * Units
 * Modifiers
 * Dangers
 * Minions

Mods can be dependant on one another which would allows for the easy creation of submods.

Assets
Assets can be added for many of the different elements. The following lists the asset types and their respective file sizes: Agents: 600x600 Armies: 512x512 Border: 256x256 Elite Units: 128x128 Events: 400x200 Factions: 800x566 Frame: 132x132 Heraldry: 128x128 Institutions: 128x128 Icons: 64x64 Lesser Evils: 600x600 Lore: 600x300 Minions: 128x128 Old Ones: 1440x1080 POI: 256x256 Political: 1000x667 Quest: 128x128 Portraits: 81x130 Regions: 340x256 Rivals: 600x600 Scenario: Mixed

There are also Music files, Particle Effects and Shaders, but these don't have a specific resolution.