You can customize almost every detail of the game menu's 2d graphics, this guide is focused on "Theme" files, wich include:

• background image
• main menu logo
• loading screen
• briefing image

Other menu details can be found browsing the game directory, specific names and locations for these files won't be listed in this guide.

Since this guide is all about replacing files in the game installation directory, here some basic advice:

• You'll need to overwrite original picture files, so it's reccomended to backup the files you intend to customize, otherwise they will be replaced and lost unless you cancel and reinstall the game.
  
• You may want to backup your custom pictrure files as well, since any automatic game update could delete and replace them.
  
• Correct file pathing and renamig is obviously crucial, though not case sensitive.

Customizable picture files are in .png format, wich is very common: conversion from other formats is easily achieved with any editing software (even Paint!) and plenty of free online converters.

• Background and loading screen: unrestricted
  your image will be adapted to fit your screen resolution, if it's bigger will be centered and exceeding edges will be cut off. The image won't be distorted in any circumstance.
  
• Briefing image: 1:1 ratio
  your custom briefing image has to be square, otherwise it will be distorted.
  
• Menu logo: 256x128
  if your custom title image has different size it will be aligned top left of the title position and cut off if exceeds its size. The image won't be distorted.

You have to name your files depending on their destination:

• "base-menu-window.png"
  replaces a background image.
  
• "MainMenulogo.png"
  replaces a menu logo.
  
• "loading-window.png"
  replaces a loading screen.
  
• "briefing-map-default.png"
  replaces a briefing image.

DCS and each of its mods have its own theme, wich can be selected via game menu > options > misc > theme
Each theme corresponds to a theme folder in the game installation directory (listed below),
this folder (usually named "themes" or "skins") should include at least the four files listed in the previous section: if one is missing or deleted the game will load the correspondig file from the DCS default theme folder.

replacing "briefing-map-default.png" in the default theme folder won't affect the main background, still the game will load this file if it's missing from a mod's theme folder.

Here's an example showing the open folder location and the in-game results. (SU-25T theme)

here's a list of default locations for the game's theme folders, it's based on a Windows 7, English language, standard installation via Steam, paths may vary if your settings are different.

• Default theme folder path:
  C:\Program Files (x86)\Steam\steamapps\common\DCSWorld\MissionEditor\themes\main\images
• "SU-25T" theme folder path:
  C:\Program Files (x86)\Steam\steamapps\common\DCSWorld\Mods\aircraft\Su-25T\Theme\ME
• Most mods' theme folder path:
  C:\Program Files (x86)\Steam\steamapps\common\DCSWorld\Mods\aircraft\<<YOUR MOD>>\Skins\1\ME
• "Combined Arms" theme folder path:
  C:\Program Files (x86)\Steam\steamapps\common\DCSWorld\Mods\tech\CombinedArms\Skins\1\ME