PhotoShop is my weapon so that's all I can offer.
For the soot, I would drop blackish-brown on a new layer above the floor using a noise filter with a high density right around the mouth of the fireplace then use the smudge tool to smudge it in a realistic way. Consider a broom or rags being used to 'clean up' the area.
Hay in the horse's area can be done with symbols, or a custom brush set to a straw color. There are actually a couple brushes in PS that would work, but I don't know if Gimp has such brushes, but considering the size of the community I'm sure someone has one that would work.
All the stains would just be transparent layers over the floor and table layers with a grey fill set to burn or dodge with an opacity of about 10 to 25%. Whatever looks good.