RE "better way to express it":
First, as Karthik says, the conventional phrase is, "A picture is WORTH a thousand words." But if you use the standard phrase, it's difficult to see how you could add "in a fraction of a second". "Worth" doesn't "happen" like "says" does.
To go back to using "says" for a moment: While your meaning is apparent, your examples are awkward. You have two adverbial phrases, "most importantly" and "in a fraction of a second", stuck together, which makes the connection to the object or objects obscure. Phrases like "most importantly" are routinely used with no object, but combining it with "in a fraction of a second" gets clumsy. You could say, "A picture says a thousand words in a fraction of a second". Note there shouldn't be a comma in there. But it's not clear where to add "most importantly" without gumming it all up.
I suppose the grammatically correct way to say it would be, "A picture says a thousand words, and the most important fact is that it says them in a fraction of a second." But that's not very catchy. My inclination would be to write, "A picture says a thousand words, and most importantly, it says them in a fraction of a second." Then "most importantly" has no object but you get away with that, and "in a fraction of a second" clearly modifies the second "says".
Now if you substitute the "correct" phrasing, "worth", the sentence works: "A picture is worth a thousand words, and most importantly, it says them in a fraction of a second."