With Gmail's web interface, it's more complicated than that. Images are loaded by default, but only through a proxy: Google's servers go fetch the image for you and then serve you the image. They only do this once you open the email. So you do leak whether you opened the email, but not your IP address, browser headers, etc. Here's a blog post from 2013 about it: https://blog.filippo.io/how-the-new-gmail-image-proxy-works-...

That's what I've always heard and I can only speak to my personal experience on this, but we track open rates on our marketing emails and have a 36% open rate historically. I'm sure this isn't everyone who is opening them as some percentage have images disabled, but to me this says that a large percentage of our clients do have images enabled as I doubt our actual open rate is tremendously higher than what we are seeing reported.