When might it be necessary to compress a picture? when is it preferable to maintain high resolution in your pictures? when is a lower resolution preferable? how does cropping affect a picture?

Cropping affect affects a picture by taking out what you don't want in the picture. Like someone is bombing your picture you would want to crop them out. High resolution is when you want your picture to look more clearer. Low resolution is when is low quality. It's like a Android compared to a iPhone. A iPhone has a better resolution. Or quality. On the camera unlike a andriod