There is a way to do batch conversions with a plugin but I don't have that. You may be able to find that info on the support pages at the website.
But for one image at a time it's still simple...
- Open your large image with irfanview
- In the top menu bar select > File
- Then scroll down and select > Save (original folder)
- A "Save Options" window will pop up on the right side
- Select 65% and then be sure to click "Save" at the bottom
- Then Select "Save" on the left window ( your files original folder)
I just ran your photo-05 squirrel through that process and got it down to 297kb from 1.79mb
I always paste a copy of my original image folder to my desktop, from there I do my edits.
That way I still have a copy of my full size original images that have not been changed.