You cannot edit images directly from the Web Page Editor. Instead, you must edit the original image file in an appropriate application and then update the image in the web page as follows:
Image in Library
Replace the image file in the package, using exactly the same file name. Because the image in the web page is linked to the package file, it updates automatically.
Image on My Computer
Edit the image properties, replacing the Source entry with the path to the edited image file. All other image properties except size remain unchanged.
URL, embedded
Edit the image properties, replacing the Source entry with the path to the edited image file. All other image properties except size remain unchanged.
URL, linked
Replace the image on the source Internet or intranet site, using exactly the same file name and path to the image file within the site structure. Because the image in the web page is linked to the source file, it updates automatically.