In Photoshop go to
Windows->Actions
then in actions window (with your file open) clcik the folder with star thingy, call it face resize.
Then go to Image->Image Size
Tick constrain proportians then put width at 20.
Click OK then save it (as .png and with save as copy UNTICKED) exit, then click stop on the actions window.
Your action is now done. Open up all your faces, go to File->Automate->Batch, choose your face resize action, source "Opened Files", destination "Folder" then choose your folder and click OK