Hi Andrea,
Knowing where to crop a picture can be a challenge, although as a starting point, I always try to crop in the camera. What this means though, is that you do need to be aware, when you are composing, what the key elements are within the frame.
We know the hero of this photograph is the two zebras, but there is another key element that I think deserves more space, and that is those two clouds. For me, the two clouds appear to be moving across this savanna, like giant white wildebeest, they are free spirits just like those zebra, although clouds do not have to worry about a lion leaping out of the grass to upset their day.
With this image, I would have been inclined to include the top of that cumulus cloud. It would have added some elegant balance to this photograph. To make the image even more interesting, I would then have cropped the image to square, thereby eliminating some of that distracting grass to the left and right of the frame.
The only other adjustment I would then make is to use the selection tool to select the zebras, ensuring that the selection is feathered by about 20 pixels, and then I would use the curves tool to gently lighten the zebras. One you have done this, the photo should look stronger.
Cheers, Anthony