how can i protect my art work in my website?

Watermark it and/or only post low-resolution versions to it. Any actual protective measure can be bypassed with a screenshot.

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To add a Watermark, you can use Aperture's built-in export presets:

In the "Export" panel set the "export preset" to "Edit".

Then select a watermark image with "Choose Watermark" and click "Add Watermark".

Another way to add a watermark annotation is the "BorderFX" plug-in: Download it here: http://www.iborderfx.com/BorderFX