Let's say you want to assign a portrait for John Doe leader of a country whose tag is TAG

1 . Find the file in which the character is defined. Most are defined in the folder /common/characters, or in /historycountries/

2. Copy his portrait reference. Most of times, it's something like GFX_portrait_john_doe or GFX_portrait_tag_john_doe, but unfortunately, the naming is non consistent. For the sake of the example, let's say it is GFX_portrait_john_doe;

3 . Once you have the reference, you have to create a file my_portraits.gfx (or any unique name you want, but preferably not too early in the alphabetical order, so never pick '_' as the first letter) in the interface folder which contains:

Once you know how to do it, it's quite simple.