How do I prevent my pictures from being copied?and this, I regret to say, is a magic trick that has yet to be perfected. Others ask
Why does Google let people publish blogs with pictures stolen from other blogs?and here, we have to note that Google has no interest in controlling blog content, beyond the limits of their own TOS regulations.
Each of us is responsible for ensuring that our content - that we own - remains ours. Google doesn't do this for us. They aren't legally, morally, or socially responsible for controlling duplication of blog / website content, until the owner of any blog or website reports a problem. And even when a problem is reported, they have to consider both sides of the story.
Protecting our blog content is our responsibility.
Besides the fact that you can't effectively block the context menu "Save Image As" command, you can't control what's stored on my computer.
I've written about cache, here and there. Cache is a basic feature in Internet design. It lets us view the same web site (blog) repeatedly, without requiring that the same content be downloaded from the server repeatedly. If you're viewing this article using your browser, it's cached in your browser.
If your computer is on a large network, it's possible that your computer connects to the Internet through a caching proxy server. If your ISP offers enhanced bandwidth, it's possible that they cache content too.
If I added a picture to this article, it would be cached, along with the text that you are reading. The right software can search and retrieve any cached content, from any cache. Anybody who needs to "steal" your pictures just needs the right software, and he can steal them right out of cache.
Besides blocking the "Save Image As" wizard, and from prohibiting caching, you'll face a third challenge. What's on my screen can be copied. Screen printing is very popular software. Print the screen, crop everything but the picture of interest, and there it is. Save it, and you have it. Display the picture full size, print that, and you have the full size copy.
How about watermarking the pictures? Put the URL of the web site in the corner. You can do this using IrfanView, PhotoShop, and a few more image processing programs. But watermarks are just bits. What you can take out, I can put back, with more software. And watermarks make the pictures ugly. They are as bad as the broadcast TV commercials at the bottom of the screen.
Finally, you are entitled to copyright your content. There are some free copyright "protection" services too. Register your content, and it's protected. But no, the Internet reaches all over the world. Can you afford the lawyers, that will practice in every nation worldwide, and enforce your copyright? If your content / pictures are being pirated on a Blogger / Google hosted web site, you may be able to get action from a DMCA Complaint - but note the warnings, carefully.
Face it - once you publish a picture in your blog, it too is like a dandelion. It's out of your control.
Sorry. Magic only goes so far.