Spinning of the "Bring the character back to life" topic i have a question: On what does it depend that the portrait of a party member is still visible after his death or not?

If the body of the character is splattered, decapitated or totaly crushed, the portrait will disappear from the right menu. The pieces of your character should disappear soon too. If the character just dies and is just laying there, his/hers portrait will just be faded and he/she is ready for resurrection (or loading the game over, which always is my favourite ;)).

Is this specific for BG or is there a D&D rule for beeing "just" dead or "completely" dead?

It's just a matter of the different parts of the body sticking together or not. You must have noticed how some monsters just fall down when they die, and some just splatter and spread all over the place and disappear.

I'm not sure how this is in (A)D&D but I thought that you splattered when you die on a critical hit (correct me when wrong)

I thought it had something to do with the amount of damage done to you when you die. i.e. how many negative hit points you end up with. Can't remember where I got this idea from and it might be from another game.

There is also the constitution factor. If you look at the tables in the manual, there is a %chance of being succesfully resurected. Perhaps the game rolls against this automatically at death and if you fail, splatters you. Afterall its better to do it then that wait to you get to a temple and then say you failed the resurection.

Just checked the manual and it says - when hit points reach '0 the character is dead. If one of your characters suffers massive damage, he or she will be forever dead, beyond hope of ressurection.'

Of course critical hits does come into this because they do double damage so are more likely to do the 'massive damage.'

I also think that if you are to be splattered, you will have to recieve a so and so amount of negative hp. From my PnP days with AD&D, I remember that when you come to 0 hp, you are not dead but only unconsius. When you for example get -3 hp, you are beginning to get kind of dead. Therefrom one only goes lower and lower after how critical they killing strike was. If you go long enough down, the body will be kind of decapitated.

Baldur's Gate uses the optional "Death Door" rule, which means that your character will be dead but still raisable/resurrectable unless he goes to more than -10HP on death. So for instance, if an enemy hits your NPC for 12 damage when they're reduced to only 1HP, you will go beyond the Death Door (to -11HP), so they will be gone for good - their portrait will disappear.
In short - if anyone gets reduced to more than -10HP, they're gone and cannot be returned.