I had the same question not that long ago (How to post a lower resolution) and I asked one of the experts here Torstan about it:
Took me a bit to figure it out because I AM using Gimp but I think I got it now. If not hopefully someone here will point it out to me when I post something that is clearly full resolution. In Gimp there is 'Image>Scale image' where you can change the size of things as we;; as pixel resolution and so on.by reducing the dpi in the Image settings from 300 to 100 - and it automatically reduces the pixel dimensions of the image by 1/3. In Gimp I think you need to reduce the pixel dimensions yourself. I usually reduce the size by 1/3 as print is 300dpi and screen resolution is about 100. So you end up seeing it on screen at roughly the size you'd see it on paper.
edit: Ninja'd by Crayola