Detecting object

I want a software built in Python which is able to detect and focus the specified realistic object (which can be different paper cards) from different orderless realistic backgrounds in a photo. After focusing the object, the exact size of the picture containing the full paper card excepting the background will be saved as an image file (the size of it should be the same every time). Different paper cards mean that the pictures on the paper cards can be different and the color of the paper cards may be different, however, the sizes of the paper cards will be the same/similar (the paper card occupy the area of the photo will be same/similar too) and the paper card will occur at different positions in the background every time. Only 1 object will occur in a photo. In other words, the software should be able to get the same size of the images of the paper cards in different situations. (Actually, the limitations of the paper cards depend on what characteristics of the paper card you will consider to detect the object, but different pictures on the paper card is a MUST.) Thanks a lot!