I’m not an advocate of stealing and reposting others’ work online uncredited or modified. That happens way too much, no matter where or when, and needs to cease, being an egregious and otherwise lazy offense to creators of all kinds.
I’m also not an advocate of ever disabling or impeding basic functionality of internet browsers in an attempt to “save” your work from being spread. A lot of times it’s pointless, anyway; for example, Firefox allows anyone to view all media loaded on the page under Tools > Page Info, and there’s no way to disable that. Your images could also be indexed in Google searches, and unless you have redirects set for your images so that viewing them individually is impossible (which, as I’ve seen, don’t work consistently), that provides another direct route to your images. Especially if you’ve posted them in more than one place (on Tumblr or Blogger, for example).
Firefox allows anyone to view all media loaded on the page.
Users: if ever you encounter a page on the internet that has disabled the right click, place the following code in a bookmarklet in your browser’s Bookmarks Bar and click whenever needed to reenable it:
If you absolutely don’t ever want anybody saving your work to their computer, inspecting your html, or even sharing links for what is probably an innocuous purpose, then don’t post it online.