Hopefully you can create the image pointing to the left; this will make the code work much easier.

just…flip them

Actually, you want the spritesheets like this. Smaller PNG = less memory allocation. 
This is industry standard. Every 2d game engine comes with a h-flip parameter to turn the sprite around.

you could (if you need it) use a program like gimp to make a flipped version just flip each frame (horizontally)

Use Aseprite to do this. You can easily create all the images. Just use the Shift+H or Shift+V command.