Different Niantic games crop the image in different ways, but in my opinion the best results are a 1:1 ratio with the focal point of the wayspot in the centre. If possible leave a border around the object to reduce cropping, but not at the expense of including things like people, number plates etc. For example:
Not all objects fit nicely into a 1:1 space though, if it’s a difficult object then make sure that the main focus of the wayspot is in the centre - for example, the most recognisable part of a building. A common mistake is to use 16:9 ratio and then have the focal point off to the side or top/bottom, and that leads to it being cropped out.