The Dead Sea picture is not really helpful. The person in the photo appears to be alive. :) This is more like "Name a sea that begins with 'D'." In a picture quiz, the picture should convey the answer, not just be a decoration.
I think it's not an easy clue, but a clever one. Usually you would get a picture that shows something typical of a sea, like the outline or a famous city on it's shore. Here your first thought is "a picture with water? Well yeah it is a sea", but if you then think about why this picture is supposed to be very specific for the answer you can get the hint.
With its concentration in salt, much more of your body (approx 35% of its volume) stays above the waterline. For regular water, it's typically little more than your head.
Definitely, and even if it wasn't, most people can probably recognize that the sport is American football, and thus the clue is essentially just "name an American city starting with D."
https://en.wikipedia.org/wiki/Diwali
Great series btw
I guess if you know capitals, you can get Doha. And religious familiarity will get Dome on the Rock. Maybe Dubrovnik is well-known amongst Europeans.