low pass filter

How to implement a simple low pass filter for image manipulation?
By convolution or multiplication? In image or frequency domain?

You could do it using a convolution kernel in the "color" domain.

But might I suggest you have a look at OpenCV:

http://sourceforge.net/projects/opencvlibrary/

It implements all normal and common image processing algorithms and also some computer vision algorithms.

When you start using it, you just can't stop!