We are happy to accept images showing any subject, however there are some subjects which are very, very common. If you are sending images of any of these then your application is likely to be rejected unless they are exceptionally good:
Flowers
Food
Photos from moving vehicles, including aeroplanes
Portraits of people doing nothing and showing no emotion
Charts and graphs
Common symbols and gesture
Landscapes and skyscapes
Waterfalls
Simple 3D renders and texts