I want to know. I was surfing a forum (Not this one) where there was a picture of a girl with a very revealing picture of a girl. I disregarded it but every now and then when I was surfing the forum my wandering eyes caught glimpse of her and I would quickly look away. I downloaded an ad blocker and I don’t think I Commited a sin since I wasn’t intentionally going to see the picture nor I wanted to see it.
No, this is no problem. It only becomes a sin if you entertain the thought of the picture, or re-visit the forum for the sake of seeing that picture.
That’s all that needs to be said.