It's suspicious if it is not done frequently, and it is a sensitive location (e.g. a huge gas tank). Cops aren't stopping you if you take pictures of the Washington monument or the Statue of Liberty. But if something that doesn't happen normally happens, it's natural the the cops want to look into it.

Sure, the picture of the painted gas tank might be on the web, but a guy with a camera might be taking pictures of other sensitive things that are at the location, not just the painted gas tank.