If you want to protect the images stored on the server and in cache, consider using encrypted images that are displayed in a security applet. I'm surprised the option is not mentioned here because its been available since 1998.
If you want to further protect images from screen capture and printscreen, that solution is also available.
Of course there are workarounds to these solutions, but they have been the most secure solutions available since 1998 and it doesn't get better... no-one has comparable solutions despite their claims to the contrary.
Banks and online surveys use these solutions and they are the best available!
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