Hi there, if you have enabled Time Machine to back up your Mac previously, then it's possible to recover the deleted photos from your Photostream. All you need to do is connect the Time Machine backup drive to your device and then launch it to restore the photos you want.
Step 1: Reveal the folder ~/Library/Application Support/iLifeAssetManagement/assets/ in the Finder.
Step 2: Enter Time Machine, go back in time to a day before you deleted the Photo Stream.
If you deleted photos from Photo Stream on your iPhone, then go to check if your iPhone has any backup. If it doesn't have one, then there is no luck.