Well if it is a black and white image and you want to add color to it there is a plug-in named colorize that will do this.
You can get it here
viewtopic.php?f=9&t=1625
Just place it in your plug-ins folder.Restart Gimp.
When you use it -
Create a new layer above the image that is desaturated.Draw some lines on the transparent layer (colored lines), of which colors you wish to use.For instance if you want the horse brown draw a line all the way through the horse on the transparent layer in that brown color.
Run the plugin and it will use each of your color selections to color the image.
If you like you can post the image here and get better help.