The Responsive Images plugin loads images at a size that is optimal for the screen size of the device on which they’re being displayed. This helps save on bandwidth and improves the user experience.
If an image doesn’t have a given dimension, it will be set it to the screen’s dimension. If an image does have a dimension set but it’s bigger than the screen’s dimension, it will reduce the set dimension to the screen’s dimension, proportionately so if the crop value is passed in. Otherwise it will default to the set image dimension.