Welcome to Android Central! Picasa refers to photos in your Google Photos in the cloud. Picasa was acquired by Google years ago and folded into Google Photos. The name Picasa persists only on certain devices that use it to designate photos in your stock Gallery that are actually in your Google Photos cloud.
If a photo is in the cloud, it's not actually taking up much storage on your phone, except for a small thumbnail. It will only take up the full amount of space if you actually download the photo from the cloud to your internal storage. Otherwise, it's just like viewing your cloud photos on the Google Photos website. If you want to delete the photo in the cloud, go to your Google Photos website and delete the photo there (or use the Google Photos app to delete it).
If you want to keep the photo in the cloud, but just don't want to display it in the stock Gallery app, then go to Settings>Accounts-Google>[your account] and uncheck Gallery or Picasa (depending on what you see there). If you don't see it there, then go to Settings>Accounts-Samsung and uncheck Gallery or Picasa if you see it.