The program is very intuitive and easy to use. The only problem is that it's in German, but it will be no problem because the process is so simple that you'll know what you'll have to do at first sight.
You'll have to choose the folder and a list of files will be shown on screen, there, you'll select the ones you want to have their size changed. The program also offers two more options: Select all (Alle auswällen) and unselect all (Keine auswällen).
Once we have selected the pictures, you'll only have to choose the new size and then, press resize (Grosse ändern).
When you'll want to finish, just press Abbrechen.
Of course, we'll be able to choose the output folder and activate the replace file option (Bilder ersetzen).
Three steps and less than one minute, that's what you need to resize your photos.