Honestly it’s impossible.
Does anything you do mean your images are safe from download [ 
spoiler … ] Nope!
You can do tricks to make it harder for people, but if you can see an image on a page then the data being served can be found regardless of how it’s delivered. Honestly any safeguard you put in place can be bypassed and circumvented. Even if you make it difficult through various methods, even a non-technical visitor could always simply screen capture the image, etc.
When I find sites or services that seemingly offer deterrents in general, I always investigate and generally 100% of the time you can quickly or eventually circumvent or bypass anything regardless of the complexity or method.
In general however, for the casual site visitor, basic deterrents are all that is needed to stop them. However most avid users or even the most basic of developers can bypass what you put in place. In the end, “Where there is a will, there is a way”.
In addition to what @pauland already suggested above regarding formatting considerations for your images.
Somethings you can do:
- Use noscript or other methods to require javascript and redirect if initially not enabled
- Use Javascript/JQuery and CSS to disable right click, drag, copy/paste, print, and try to prevent screenshots
- Use htaccess to avoid hot linking and provide redirects, etc.,
- Use a PHP proxy to serve and hide image path/links, etc.,
- Use transparent images over all images
- Etc., etc., …
- You can also decide to pay for digital signatures and tracking from Digimarc, or services like DMCA etc.,
- Or you can just use Reverse Image Search and manually drive yourself nuts constantly all you want.
[NOTE:] This next statement is not personal to anyone, just general.
As just a general observation, not seeking an endless debate and regardless of legal or ethical considerations.
The other thing that is always interesting to me is, if you look at all the beautiful images across the web that can be easily downloaded without any safeguards by world renowned photographers (two basic examples [1] , [2] as reference). I always then find it interesting why people in general then think their images are more special and need protected on their sites.
In conclusion:
In the end, is it right to take peoples stuff without permission, no. Sadly however the internet as a delivery vehicle for Text, Images, Sound, Video, Source Codes, etc., easily allows this type of mischief / theft and so much more in a rather anonymous fashion. It essence it’s a sad state of affairs on the internet, the human condition in full display, both good and bad.