I spent just 36 hours in Frankfurt. For some cities in Europe, a couple of weeks is not enough to see a fraction of the city and Frankfurt is one of this cities. I used all of my 36 hours in Frankfurt walking on the streets and trying to find the best spots possible for pictures.
Frankfurt has the perfect balance between the old romantic style and the new technological town. This type of city makes me crazy about taking more and more pictures. It has infinite great spots.
One good tip to take great photos in any city is getting to know about the city first. If you don't live in the city you need a little of luck as well. Of course trying to go to the river's opposite margin at sunset or sunrise is a good tip. I always start my day looking for the position of the sun along the day in a city but it is not easy to know where you should be at different times of the day. I tried my best to find the best spots I could.