There are quite a few things you can do to protect images. However, if someone is determined you may not completely block image access.
1] You can add a watermark to your images. I use Thumbnailer from SmallerAnimals to do this. You can use it in trial mode of course, but the cost is only $15 and it is far beyond any other product I have ever used. These guys are real experts and their entire business is based upon image manipulation. I have been using this product for well over a decade.
2] You can degrade the image quality so that it will still look good on a website and be relatively useless otherwise. Again, Thumbnailer can do this for you.
3] You can digitally mark your images with a copyright notice. This is encoded within the image and cannot be seen. Again, you can use Thumbnailer.
4] Encoding the link. You can encode the URL making it more difficult to recognize. However, any clever user may be able to find this and still access your image. It just makes is a bit more difficult.
5] You can do something like what I do with my e-mail munges where I use random numbers and use JavaScript to create a seriously confusing but functional reference. You can break the code into elements based upon the random numbers so that no single part is ever the same and all parts are required to work. I do not have sample code for you. You would have to ask someone or code it your self, however, this is not a difficult task. You can go to my site (please forgive that it is seriously ugly) and use mungeMaster to create a sample of what I am talking about. It is an e-mail munge, but you should be able to code something similar for images.
6] You can use a PHP image proxy (lack of a better term) script. These exist. I rolled my own and it works a treat, however, I suggest doing a search on the net anyway since my code is seriously out of date. There are some rather clever ones out there, but I do not know enough between them to make a recommendation. You can use php script to protect images in your favorite search engine to research this.
7] You can can use color management within the image to limit the usability and still have the image look good on the web. I am not an expert on this so you will want to research this more. Again, I am pretty sure Thumbnailer can handle this for you.
8] You can use a table and set your image as the table data background image and then within the cell data you can use the image tag to place a 1 pixel transparent image over your original and size it how you need to with width and height values.
9] You can use JavaScript to disable right clicks. Here is a link to another question on just that: https://stackoverflow.com/questions/737022/how-do-i-disable-right-click-on-my-web-page
10] You can disable right clicks using the following in your body tag.
<body oncontextmenu="return false" onselectstart="return false" ondragstart="return false">
11] You can use .htaccess and check the referrer. Here is an example code you should confirm does what you want. You may need to customize the image list. Obviously change example.com to your domain name. Test this.
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?example\.com [NC]
RewriteRule \.(jpe?g|gif|bmp|png)$ - [F]
12] Prevent search engines from indexing your images. This can be done using various methods. You may want to use more than one or all of them.
In robots.txt, you can:
User-agent: *
Disallow: /images/
--or-- by image:
User-agent: *
Disallow: /image/my-wife-looks-like-a-naked-mole-rat.jpg
You can also place this in your HTML:
<meta name="robots" content="noimageindex"></meta>
13] Honest to God copyrighting. This may not stop someone from stealing your image, however, you can seek a copyright for your images and then put a copyright notice under the image. It does make people thing twice. Sometimes more than twice!
I am sure there are other things you can do. But this is a fairly decent list to get you started.
