Helen & Charmion made the necessary points. Others simply quote standard ways of declaiming that someone either never was in court or was not formally indicted AND convicted.
I will add that "proven guilty" is legalistic shorthand jargon to convey that some jury (or judge in some cases) has decided one way or another.

Often, the way a jury votes is not in line with the facts. (O.J. Simpson comes to mind. At least 2 members of his jury admitted that they planned all along to vote 'innocent'.)
I do wish there was a better way to describe 'not convicted' other than 'innocent'.