hi deon
in 'settings' → 'external tools' → 'image processing tool' you should insert 'gimp' as your preferred image editor.
after that a right click on the image and 'edit image' will open the source image in gimp.
of course, you need to export the edited file, as 'save' would produce an .xcf file, which scribus can't read.
if you overwrite the old file, scribus will display the edited one.
in scribus 1.4.6 there is a bug which prevents gimp from loading the image if the program is already running – you have to quit it first. this is fixed in scribus 1.5.3…
utnik