Do que is kind of like a stand in for a direct object. In English, when you say "actions say more than words", 'than' stands in for 'what' is being said. When you have implied objects and subjects, they are almost always singular, which is why you say "it is raining" rather than "they are raining".
In this example Pictures say more than words, there is an implied direct object, as if to say "Pictures say more [what] than words". The implied objects are almost always singular therefore 'do' is more appropriate. If dos were to be used- it would refer the subject(s) (Fotos and palavras) which would not replace the implied direct object and would make no sense.
Hope this helped :)