Well you must be saying, how do they know what to download or upload....depending on you point of reference (confused yet? lol) Well Curiosity does the same trick your camera does. When you look at the pictures in your camera, you are actually looking at the "thumbnail" (a low resolution representation)of each photo. The rover just sends thumbnails from which decisions are made as to what gets sent from the rover. Another trick they use is one I have harped on for years (people that have taken my photography courses have heard this one before) Megapixels are a myth, you don't need them. Yes, I have heard the arguments, more megapixels clearer better pictures. True in part, but unless you are printing wall sized pictures, you many never see the difference between 6 and 16 megs. Most people never print a picture so anything over 1 megapixel is a waste and to prove that point Curiosity's cameras for the most part are on 2 megapixels! Oh I know...sensor size and all that, but still it is vastly smaller picture which takes up little space on the rover's memory cards. But most importantly is the kind of hoops NASA/JPL controllers have to jump through each day just to get data back and forth!
You really should read this article here about the cameras and what they do. It is fascinating material. Go here for more on the Economist