Well now.... let's see what we can do. If you have easy-debian working and if easy-debian gimp works then here is what you do.

We will use jpg's here as n900 probably will collapse under tiffs. Whatever, the same principle applies to any format that gimp can handle.

Take a flat field photo with fcamera as outlined in cornerfix. Get it as light as is possible WITHOUT the centre being overexposed.

In gimp open the pic you want to correct as a layer and open the flat field jpg as a layer (i.e. 2 layers in one image). Choose "divide" for the layer mode and the colour and luminance vignette should disappear. Flatten the image. The image will lighten in the process but colors -> brightness and contrast, or colors -> levels should take care of that.

What I don't know is whether the gimp version available in easy-debian will do this, but I think it will. It certainly works on the desktop.

This is a link to a set of routines that include a flat field subtraction script for Gimp. Copy flat_division.scm from the archive to a Gimp "scripts" folder (check under edit->preferences->folders), refresh scripts or restart and thew routine will be available under filters->astronomy.

This is an article on a more manual approach using layers. It is for Photoshop but applies to Gimp as do the principles - gets one thinking etc.