Plug your device int your computer and look for the Icewind Dale folder. It will look something like: This PC\Fire\Internal storage\Android\data\com.beamdog.icewinddale\files\portraits
The first part of that path will vary depending on your PC and Android device, but the part starting with "Android" should stay the same. The "files" folder should have your baldur.ini in it. The portraits folder may not exist, create it if so. Then just put the portraits in there just like on the PC version.
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This PC\Fire\Internal storage\Android\data\com.beamdog.icewinddale\files\portraits
The first part of that path will vary depending on your PC and Android device, but the part starting with "Android" should stay the same. The "files" folder should have your baldur.ini in it. The portraits folder may not exist, create it if so. Then just put the portraits in there just like on the PC version.