Use JPG instead of PNG for sure help. If you can use smaller size image other than that I dont think you can do much to decrease the memory. Im not sure if the link you share will help but may I ask why you want to decrease the texture memory?
You can also use tools like redketchup to compress, resize, convert, etc. all kinds of graphics! Very useful if I say so myself, the difference in quality between the images before and after is like, barely noticeable.
Edit: just saw you asked about the compressor in a reply above, and yes! It’d work. Personally, I still prefer redketchup since it has a wider variety of tools and options.