I've seen similar complaints but not quite the same, so I figure I should make my own post. Most of the other complaints are about uploading images from Safari to your own WordPress, not viewers of your site.
I've been using the same theme for over a year now and this has just become a problem. I was also using an older version of WordPress as I was paranoid to upgrade and break anything! So this started happening when I didn't change anything (that I recall of course). I hadn't added or upgraded any new plugins, etc.
A few weeks ago, a Mac user told me that some of the images on my latest blog post were mostly gray boxes. I was on my iphone at the time and sure enough, they were. By the time I got to my PC to work on, the images were there and I thought it was a server snafu. But since then, I've realized that random images appear partially gray (or partially loaded) for all Safari users (PC and Mac) and for all Mac users (Safari or Chrome tested so far). All images appear perfectly fine on my PC with Chrome, IE, and Firefox.
Again, this just started happening even though I don't recall anything changing. I've since upgraded WordPress and my active plugins to see if I could "fix it". The images from older posts are also affected. And it's random - you can refresh the page and different images appear gray than the first time.
(as a side note, Safari for PC has all sorts of issues with the blog - doesn't recognize the images being centered, line breaks or the fonts!)