Worth-it to optimize image file size for a website?

Question

I found a program called png optimizer that reduces the size of png, gif, bmp, or tga files without changing the appearance of the image at all. I was testing it out by downloading a web-page and then optimizing all the images on that page and seeing what the size difference was. If you look at the properties window for each image, optimized and not, some images are reduced in "size", but the "size on disk" remains the same.

If you want more information on "Size" vs "Size on Disk" then look here https://stackoverflow.com/questions/15470787/please-help-me-understand-size-vs-size-on-disk.

I'm wondering if the difference between the "Size" and "Size on Disk" properties make any difference for how much data is transferred over the internet when a web-page is viewed. Basically, is it worth-it to reduce the "size" of an image for viewing over the internet if the "size on disk" remains the same?

http://www.pngoptimizer.com/

Answer 1

Basically, is it worth-it to reduce the "size" of an image for viewing over the internet if the "size on disk" remains the same?

Yes. The "size on disk" is only relevant for evaluating disk occupancy. To measure how many bytes are transmitted to the wire, the size-on-disk is totally irrelevant, what matters is the real size. So, any reduction in real size attained by a PNG optimizer counts here.