Keeping pixels of the previous frame?

Hi,
it seems that usually an image at output of a module at a frame is computed by the current images at the input(s).
Is there a possibility to keep/save pixels which once appeared at the input and every new pixels at input for every frame are then added to the modules (output) image?
I did several tests and read descriptions e.g. transform, merge, ... but such operations / settings are not mentioned.
I also tried paint because the description sounds like a real creation of an image but I also got only current input images at output. And I think I don't need strokes but something like merge current whole input image into/with my last output image for every frame.
I hope I described it understandable.

Thanks!

Hi,
are there no ideas/possibilities or is it not understandable?
For instance you want to use particles to fill a shape. So you need an appeared particle only in its origin appearance position and it can be killed (saving cpu, ...) if you "capture" and hold it in a picture. And every new appearing particle will be merged into this picture for every frame.

Look at the Delay option on the Time Speed tool.

You're looking for the Trails tool.