Distances In a Single Image With Some Real References
Well, you can have an approximation. Due to the forms in the image are non regular, completely plain, its hard if not impossible to know the camera distortion. But if you know the size of the wheels and also you have the measures of the stones you can approximate the distance of the cart to the stones by using this equation:

$$
D_w = \frac{S_w * f}{P_w},
$$

where: $D_w$ is the distance from the camera to the wheel, $S_w$ is the real height of the wheel, $f$ is the distance of the focal to the film when the film is in the camera (You need to search for this parameter to use this approach, also you need to know the proportion of the real impressed picture with the film) and $P_w$ is the height of the wheel in the film. The same thing for the stone:

$$
D_r = \frac{S_r * f}{P_r},
$$

where: $D_r$ is the distance from the camera to the stone, $S_r$ is the real height of the stone and $P_r$ is the height of the stone in the film. Finally the distance $D_t$ from the wheel to the stone is:

$$D_t= D_r -D_w$$

I recomend you to use the objects that are marked in red in the next image, because are the nearest to the epipolar line.

Hope this could help, Best.