kombinating blender renderings with other pictures

okay, here is my question:
so far i want to kombinate effects rendered by blender with other pictures, i used gimp to kombinate them, and when importing the blender-pictures i used to set the transparent-mode for the imported picture, to make the darkness-depth to transpatent (f.e. black=100%, grey 50% transparent).

But now i enforced 2 problems:
first, partikel system seem not to get enough darkness to paste them as transparent layers into my pictures and seem to draw in the rendering something like a border, how can i disable this?

second, i am going to render animations for flash videos, ok i can import the picture as static background, but first, this means my application grows to big and second, when the background is animated too, this is not a solution. well i am not sure if png kann get exported as an animation and as a transparent background, but i am sure, i cannot export with png a “transparent-field per darkness-depth”, so anyone got an solution for this or know any programm to edit animations for transparentmodes, because with gimp i cannot edit animations.

thx 4 any help and sorry for my bad english, i am german …

Your English is better than my German!

Have you looked at CinePaint?
http://cinepaint.sourceforge.net/

You might see if it can do what you need.

It’s an offshoot of the GIMP used for movie productions.