Ah, that makes sense. Wasn't sure of the context. Images that are your own, meaning you photographed the subject with your camera or created the image with an art studio, or out and out bought the rights to, are called proprietary assets. Images we have permission to use usually come with the requirement that we host them ourselves and sometimes attribution on the page(s) where they are used. A fair arrangement that lumps this group in with the rest, mentioned above.
Proprietary assets we would host on our own domain so we never give up ownership. The definition of Public Domain is not on the tip of my tongue just now, but I have always been of the belief that if one publishes a photo on a publicly accessed web site that is not their own, and does not clearly mark with a copyright symbol, year and owner either directly or in the footer credits, the photo falls into the Public Domain. This bears further research, but signals how we must protect what we own and value.
There are many ways we can protect our images, copyright ownership being at the head of the list, but also how we publish them. For starters, upload them to an
assets folder, not an
images folder. Then in
robots.txt disallow access to the
assets folder. Google and other search engines look for the
images folder. The pictures that you don't want indexed go into your assets folder. The ones you do, go into the images folder.
That buys some obscurity and helps prevent the URI's from leaking out onto the internet via search. Google, et al will be content to crawl your images folder and depending on the importance they deduce in the page(s) where the images appear may index them. Now they will come up in image search results, and where most people snag them.
Only put pictures on your site that you own, the exception being those used with permission or drawn from (and confirmed to be in) the Public Domain. Never hot-link to another domain unless it is intended for that purpose. Image hosting sites are not Public Domain. The members who use the service still maintain ownership of their images. To use their pictures one would still need permission, and then hot-linking is okay.
Hot-linking steals bandwidth for the hosting domain. It may seem innocent enough, but it is theft. That's why permission is so important. If the host doesn't care about the bandwidth, permission can be easily granted. If they do care, and they still give permission to use their image, then you host the image and pay for the bandwidth. Fair arrangement all around.
Where the harm of hot-linking can really add up is when a page goes viral. A million or more hits a day, or 10 million, or more, means the images are served out that many times. If there are multiple images in each request, multiply the damage even further. Ten 100K pictures is 1MB. A thousand hits is 1GB. A million hits is 1TB, so 10 TB would be a huge hit on that poor person's otherwise relatively quiet server. Making sense?
Another thing about published images that prevents theft is quality. The lower the quality or the smaller the image, the less likely it will be scraped. Nobody is going to steal a 400 X 300 JPG with 70% quality. But a 750 X 600 with 100% quality (no compression) is tempting, especially if it is a compelling image.
Now if you want to display a high quality image, that's okay. Just don't do it on a landing page (one that is referred from SERP's or other sites, FB, etc.). Make the visitor click their way to the secondary page where the image can be found. Be sure the page has a NOINDEX robots directive in the head meta data.
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
This will keep it from getting indexed. The lower quality one on the landing page is the one that will be indexed. If you want to keep people from right clicking the image, put the image in with an absolutely positioned, transparent layer above it. Now when the user right clicks the image there won't be a reference to the image properties or URL. Again, though, do some compression to scale back the quality. 80% is plenty good. And it saves on your page load times.
I think this is going to be a lot of information for you to digest, and hope that I at least answered your question to some degree, what with all the prattling on about assets.