In the property window of an image object, you will find a "Setup" button that allows modifications to the image like rotations, cropping, application of transparency, etc. These modifications are applied to the web output only. In no case will the original image be modified. When you apply those modifications to an image, Web Creator will create a new image (possibly with a different extension) which includes all modifications. These modifications will sometimes change the image size (in Kb), making them smaller most of the time.