Are you using Irfanview?
If so, after you've done the resizing, and clicked 'OK', you need to go back into 'File', select and click on 'Save'. Another window will appear, click 'Yes' to confirm. A second, smaller window will appear, asking if you want to replace the file etc., click 'Yes' if you do. You can check to see if the 'Saves' have been made, by going back into file, and selecting the pics - if all has been done correctly, they will appear at the last size selected.
Hope this helps.
Terry.