As long as you haven't saved anything, or downloaded ANYTHING since the deletion, recovery should still be possible. When your Mac deletes something, it doesn't really delete it as much as it tells your computer to recycle that space for new data. If nothing new has been saved, those sectors will still be holding the picture data. Mac Data Recovery will look for that and try to restore your library.
Mac Data Recovery can also use two types of file search depending on what kind of volume it was deleted from. If it was deleted from an unjournaled volume (like an external drive or secondary internal), it will look for the file signatures, specific identifying characteristics of that file. If you deleted the pictures from your start-up disk, then it will look using a journalled search. Your Mac startup disk uses a journal to track changes to files and folders as you work.
Finally, when you install Mac Data Recovery, make sure you install it on a different computer than the one you are trying to recover from. It won’t work and it may cause more damage if you try to install it onto the drive you are trying to get your photos back from.