Hi Sally,
Thanks for sending in this photo of a Cheetah. It is a reasonable portrait of the animal and about the only aesthetic change I would make to the image at this level is simply to ease up on the vignette, the darkening of the corners that we can see. Your concern in writing to me though has more to do with exposure management and whether to use the Time or Aperture Priority setting for your photographs.
To determine what to use it is first important to determine what you are trying to achieve in an image; is your priority to control the depth of field or are you more interested in freezing (or blurring) action. If your priority is depth of field then use the Aperture Priority setting to manage the exposure; if you want to manage movement, use the Time priority.
Whichever setting you use, it is still important to keep an occasional eye on what the other numbers are doing… if you are outside in bright sunlight working in Aperture Priority mode with the ISO set to 6400 ISO and the aperture at f2.8 then chances are you are going to get an over-exposed image; the automatics are not going to cope. The same with working in dark conditions; if set your ISO to 100 and shutter speed to 1/4000th of a second then you will almost certainly get underexposed images.
With this image you made the photo at f5.6 at 1/800th of a second at 3200 ISO. I would have been inclined to pull the ISO back to 800 ISO; in this particular situation you could then have used a shutter speed of 1/200th of a second with room to spare. Pulling the ISO down will give you better overall image quality although for the moment this photo is acceptable. Because the cheetah is not moving you do not need a fast shutter speed… 1/100th of a second would even be good enough and finally, the wide aperture of f5.6 (the widest aperture on your lens) means that this will get the background looking soft.
Hope this is a help… Anthony