How do I find pictures on my iCloud that I deleted from my phone?

If you had enabled iCloud Photos and then if you delete Photos and Videos from the iPhone then it is deleted from iCloud Photos too, and vice-versa. Also, it is deleted from all the devices that use the same Apple ID.

And if you happen to take a backup either over iTunes or iCloud after deleting Photos and Videos all the photos are deleted from the backup (iCloud/iTunes) too when you take the backup next time.

I never understood why it's set up that way. To me the point of uploading photos is to keep them stored so I can then delete them from my device and free up space.

There should be an option that asks: delete from iCloud also? or something like that.