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?

You might see if it can do what you need.

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