The Compress Image feature is active when an image is selected and you want to change the data size of the image, at the expense of a optional and controllable loss of quality set in a dialog box.
|The Compress Image dialog|
Compressing is useful if you insert a high resolution image in a document. For example a 4000x4000 pixel image in bitmap format (BMP) is approximately of 48MB size, which can turn you document edition hard in constrained-resources computers. By compressing the image with the right format and quality index you can reduce the size of the picture with no loss of visual quality and have the document file much lighter to edit.