Firstly, I assume that the files don’t have an extension as per PatCooke’s post/suggestion.
I’ll explain the ren (RENAME) command in meticulous detail, but if you don’t understand it, I suggest you get someone to help you, in case things go wrong in the renaming:
1. Open Windows Explorer and when your folder name appears in the main listing, highlight it, hold down the Shift key, right-click on its name and choose Open command window here.
2. You should now have a black screen (command prompt) showing as per your screenshot. If your folder name is not showing alongside the prompt, you have opened the command prompt at the wrong location.
3. Now for the instructions to type alongside the > on the screen.
4. Type the word ren followed by a space followed by *.* followed by a space followed by *.jpg followed by pressing the Enter key.
5. The above command will rename everything (*.*) in the folder to *.jpg (i.e. adding .jpg to all the files).