Simply go to the Settings tab > SEO > Footer, and paste the following:
<body oncontextmenu="return false;">
Save this change and it will be active when you next Publish your site. This option does not work on every web browser, but it does work on some of them.
But one understand that that it is virtually impossible to fully protect any image on the Internet. If someone wants it, they will find a way to access it unfortunately. Pro users have the option to use password protected pages which offer the best form of protection, though visitors would then need a password to view the images.
Rather than protect images in this way, some photographers and artists choose to use a graphic program to add a watermark or stamp onto the online version of their photos. Although it does not prevent the photo from being downloaded, it does protect the photo in most cases since the watermark renders it useless.
Most folks who download an image from your site are doing so because they like it and want to share it with others. Use this an opportunity to promote yourself by adding a watermark that shows the address of your site. Then anyone who sees the image elsewhere will know you took it and likely visit your site.
source: Weebly Help Center