Is a Muslim sinful if pictures of living beings are found in his house, whether hung or hidden, and the father refuses to remove them despite being advised to do so?

The types of images and the ruling on each type have been previously explained. The preponderant opinion is the permissibility of photographic images, unless there is something in them that makes them forbidden, such as being of a woman who is openly displaying her adornment (mutabarrijah) or if they are intended for veneration. This is because they are not an imitation of Allah’s creation, but rather a manifestation of what He has created. It was also previously explained that it is not permissible to hang photographic images of living beings due to the veneration this can lead to. You have done well in advising your father to remove the hanging pictures. It is recommended to be gentle with him in explaining the ruling of prohibition and to seek help from someone he respects to advise him. If he insists thereafter, you have fulfilled your duty and no sin will befall you.