@Denise20 Do not worry I think we have got it down to what the cause is now From what I think has happened is that in the gallery app you are viewing pictures that are stored in the cloud (via Samsung Cloud maybe) So in the Gallery app if you go to Settings via the 3 dots in the top right under Samsung cloud you will see the email you use, and what is happening is that the pictures you see are on the cloud not on the device like what you have just seen so, I think you have a new phone? if so you will need to download the saved pictures (From Samsung Cloud) to the phone's internal or SD card if you have one. All pictures you take now will be saved on the phone as we changed the settings for that already. If you do not understand any of that please do feel free to carry on asking questions.
Also you can view the online photo's here https://www.samsung.com/uk/apps/samsung-cloud/