disk image privacy

I have created a disk image - Image from folder - to keep some sensitive files. I have also set the password. The problem is when I open this image and then if another user logs in while it is still mounted, he can see these files. I also changed the privacy setting thru Get Info to None but still at least once another user can open the folder and browse files. But second time when he tries to open the folder, it does not allow. I think that this is a bug which is exposing files to unintended users. Anybody else has faced this issue or this needs an urgent fix.