Lower quality picture after PDF encryption

 Can someone help me to keep the same quality of an image (probably created on powerpoint and converted into pdf) after encryption? I am using the security option  and gostscript 128bit encryption, chosing to disallow copying and modifying of content and inserting an owner password only. There is some grass in the picture and after encryption you cannot distinguish the single leaves any more.

Do you have an idea of why this happens? Thanks in advance. Federico