The crosswalks Captcha provides the most difficult cultural discrepancy for me. Some US crosswalks look like European "forbidden for all traffic" road markings. Pretty much the opposite of a crosswalk. I always have to do a double take on those.
Some of pictures are the kinda that hang suspended from a cable. We just don't have those in Australia so I tend to miss them.
Not sure I've ever seen a real parking meter. (Just ones in cartoons like the Simpsons). I kept getting mixed up with what was a parking meter and what was too ambiguous to tell. (Like could be intercom or a letter box, they pictures get blurry)