Find an image that you like and crop it to make your character really special
For player character portraits only one file is now required, the file ending is not important (S, M & L are all fine). Any size of up to 1024px works in theory, but to assure that the image doesn't become blurry it should be a minimum of 169x266px and the side ratios need to be the same as the ones mentioned above, otherwise the portrait will be distorted. It needs to be a .bmp in 24 bit colour depth and goes into the 'portraits' folder in the documents folder of the desired game (i.e. C:\Users\[Your Username]\Documents\Baldur's Gate - Enhanced Edition\Portraits).
Replacing companion portraits works just the same as in the originals. The override folder is also in the game's installation directory (i.e. C:\Program Files\BeamDog\Games\00783\Override)