According to those who believe in god, since "he" is a spirit and not flesh, he is indeed invisible to humans. I guess you can argue he makes himself visible through things like angels and other manifestations, most famously through Jesus Christ.

With the "god created humans in the image of god" part I think they mean more attributes like morality, reason and so on, less physical properties. In the end, humans are visible, finite beings, god is a spirit, so our visibility to each other reflects our created, embodied nature, very distinct from god's invisible, infinite nature.

Or however it goes, I'm an atheist myself so I'm maybe not the best to answer here, but I've been involved in the church for as long as I can remember in some way or another, and an eager reader of the bible, so hopefully I got the overall ideas correct :)