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Yes should be done, there's an article on Neil Patel's blog where he makes a test and sees a difference on rankings. You should check that. Just don't do random comments on random blogs...

I tried searching on http://www.google.com for comments placed on the disqus tool and they don't show up. Just a piece of javascript.

While i agree with Zkwarta, i would not dismiss allimages so fast, look at each image and give it an honest alt tag, do not stuff but i amm sure thet some of them will allow you to get a keyowrd in there somewhere, even if only slightly relative