How to get pic that I deleted

Look in the Album called Recently Deleted and recover it.

If you already removed it from that album, or 30 days has passed by, then the photo is not recoverable.

If the photo is more important than anything else on the phone then recover the whole phone from your last backup, but you will lose everything else since the backup.

In future you would be safer importing the photos to your computer and syncing back to the phone, then backing up the computer.