Q:

How to smooth a scanned image using Gimp

I have drawn an image which I then scanned into the computer and saved as a png. 
I just want to make the lines smooth on the image. The image is a logo 
Any help is much appreciated. I am using GIMP 2.8

A:

In order to make lines smooth, you have to vectorize the bitmap in applications like Inkscape:
You can import the image and trace it:

After the tracing operation the image is vectorized and you can work on the nodes. For example you can simplify the path, which is an automatic operation:

In this example the left image is the traced one and the right image is obtained simplifying the path (less nodes and more smooth). Obviously after this operation you can work on single nodes in order to adjust the details (in the example the eye of the lion needs a little work).
In GIMP you can blur the image in order to soften the edges, but I suggest you to vectorize the image.

A:

If instead of going to a vectors app, you want to deal with pixes, inside GIMP, you can try these simple two steps, selct:filters->blur-> gaussian blue  - try using a radius of 2-4, and then use colors->curves and pull the curve in an "S" shape to sharpen the borders back. The jaggies will smooth out, and you can fine tune how smooth you ant it with the curves.
The default curves settings will work for black over white or vice-versa - selet the appropriate channels to affect your drawing.

