When undertaking a render you need to relate it back to the real world. With a source image you effectively have a large canvas picture that can be placed in the background of a scene. Hence you cannot create any effects such as casting a shadow on the image via rendering. With the sun visible in the background image you need to duplicate the light source in the render to match the background.
The image below is a render of a character with no background overlaid on the background image you provided.
View attachment 166044
Note: The background is not ideal as the perspective is quite high. In addition with the light source from the back, it is difficult to render a clear character. Add to that the presence of the hay bail, you can only place a character on the right side of the image, or else you would have to simulate the shadow from the hay bails as well.
PS: With more practice images such as the above becomes relatively easy. Above is about 10 minutes work including render time.