I need to know, can I use images that Celebrities tweet or post on their FB pages on my blog without any copyright or legal issue?
Just because someone posts an image on twitter or facebook does not make that image free to use, so you certainly could face a legal issue. That being said, I think it is fairly commonly expected nowadays that an image posted on facebook or twitter will be used by others so I would not expect problems from it - but purely as a matter of law, using them is not "without any copyright or legal issue" as you asked.
Like everything else on the Internet, unless there is something that says you can use it or that the image is in the public domain (by a reliable source), then assume you need permission you don't have to use it.
http://www.quora.com/Copyright-Law/If-I-take-a-picture-post-it-on-my-Facebook-page-and-someone-else-takes-it-and-uses-in-some-kind-of-commercial-project-is-it-legal-to-do-so-or-the-borrower-should-have-my-approval-Its-a-FB-public-profile-and-there-are-no-legal-disclaimers-attached-UPD-Just-pulled-this-from-FB-terms-and-conditions-4 Code (markup):
Short answer: No. Long answer: You can try and hope you are lost in sea of people doing it without said celebrities caring.