I don't think it's sawdust or anything on the floor. I think it is simply a poor quality photograph and it's morning sunshine flooding through the window, the hook shape which is suggested to be a rifle butt impression is simply something on the window sill or on a shelf casting a shadow.
That's what I think anyway.
There's also one of Mikes other suggestions, that there are two bullet casings in the photograph.
Which I also disagree with.
The sun rises in the East and sets in the West.
This picture is taken by someone with their back facing North and camera lens facing South?
If so, East is to the right and West is left?
That would be consistent with this explanation.
If it's a reflection from a window, why is the reflection so small? What's obscuring the rest of the light and why can't we see some of the reflection from the object obscuring it?