How to pad an image with extra transparent border

[rinaldop]

Hello all,

I am back with a continuation of my project from my last post. I have 60 images that start at 420x37 and decrease in length by 7 pixels per image like so

   
   
...
   

What I need to do is make all of the images back to 420x37 by putting a transparent edge on the left side like so
   

I am on the right track using canvas resize with an offset to move the image to the right. Everything looks fine in Gimp but when I try to flatten the image so that I can save the new file I get an error.

Also is there a way to automate the process for all 60 images?

Thanks for helping out a newbie!