Well said Rage4Christ.
It is so easy for anyone to slip into sin. We are all capable of falling
God is not concerned about the act of sin but the reasons why we sin.
I'd be praying for God to shed His light into this area of your father's life.
I have been in that situation of being addicted to images that you describe.
Glory to God ! Actually I was addicted from as early as 8 years old, but now set free.
Addiction is always related to filling a need of the heart.
This need is always associated with the desire to be loved.
You may find that your father has a particular area in his life that he feels unloved in.
Pleasurable experiences (drugs, alcohol, sex) covers over this need, it gives the impression that the need is filled and satisfied.
But because it doesn't completely fulfill this need, we keep going back to the pleasurable experience to 'fill' the need again.
This is how addictions come about.
God is concerned about healing your father of his need to be loved in whatever area it may be, not that he is sinning.
When God heals your father, and your father feels loved in this area, he will stop sinning.