The phrase is actually "words can't do it justice". It's not the same as "a picture is worth a thousand words", but it can be similar. "A picture is worth a thousand words" means that it's better to describe something with a picture than to try and describe it with words. "Words can't do it justice" means that no matter how you describe 'it', the description will never match the impression the real thing gives. It can be similar to "a picture is worth a thousand words", except that saying has more do do with the accuracy of the thing being described rather than the impression it gives. You can equally say "pictures don't do it justice". For example, I can spend You can't say "words can't do it justice" unless I know what 'it' is, while "a picture is worth a thousand words" is a general saying.

"word can't do it justice" maybe it means that word can't bring you justice, you can't use only your words to justify yourself, you have to act, to have evidence etc... "one picture is worth a thousand words" word sometimes is not the ultimate tool to express what you think, what you want to say, you have to show something-"picture" in order to make the others understand you... if this could help you in some ways :p