Opening is a very interesting theme. This is because it makes you think outside the box. You have to search not just for windows but many other stuff. You could look deeper into each element of what you've chosen:
Windows : different landscapes outside, people outside
Doors: closed, opened, different doors such as cathedrals or castles compared to houses and cottages
Locks and keys: tattoos involve locks and keys quite a lot, symbolism, trapped, freedom
Windows : different landscapes outside, people outside
Doors: closed, opened, different doors such as cathedrals or castles compared to houses and cottages
Locks and keys: tattoos involve locks and keys quite a lot, symbolism, trapped, freedom
Opening is based around pictures like this. It shows any way of opening or entering somethings.