I think you'll need a savegame editting utility such as ShadowKeeper to change the portrait of an existing character. If you cannot select your portrait from character creation, it's either an invalid file format (has to be a bitmap file, IIRC) or there's something amiss with your BG2 directory tree (portraits folder must be in BG2 folder). You do have a large and a small .bmp of the same portrait, yes?
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Say say, oh playmate
i cannot play with you
my dolly's got the flu
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