The "werden" as auxiliary verb (both for future tense and passive voice) should not be confused with "werden" as main verb (= to become). As an auxiliary verb, it has totally lost its meaning, in the same way the auxiliary verbs "haben" and "sein" have lost it. Attaching a meaning for it makes as little sense as trying to explain "I have come" by thinking about ownership in some way. Also, keep in mind that there are two kinds of passive voice in German: Das Vorgangspassiv (werden + participle) and Zustandspassiv (sein + participle). So: Das Bild wird gemalt. = emphasis on the action. (English would use the continous form to emphasize the action: "The picture is being painted." But of course the Vorgangspassiv doesn't express continuity at all.) Das Bild ist gemalt. = emphasis on the result, e.g. to distinguish it from a photo. The same principle applies for the other tenses. Imperfekt: Das Bild wird gemalt -> Das Bild wurde gemalt. Das Bild ist gemalt -> Das Bild war gemalt. Perfekt: Das Bild wird gemalt -> Das Bild ist gemalt worden. Das Bild ist gemalt -> Das Bild ist gemalt gewesen. "Ist" instead of "hat" is used as auxiliary verb, because the choice of "sein/haben" as auxiliary verb only depends on the verb that is modified: "Ich bin geworden" und "Ich bin gewesen", but not "Ich habe geworden" und "Ich habe gewesen". Compare "Ich bin gegangen" instead of "Ich habe gegangen" for verbs of movement. Again, it's just grammar. There's no meaning whatsoever in the choice. See also: http://www.canoo.net/services/OnlineGrammar/Wort/Verb/Genera/Zustandspass.html

Firstly, "Das Bild ist gemalt" is meaningless. All pictures are painted. That is their dfinintion. Anyway: Das Bild wird gemalt = The picture is being painted. "The picture is painted" is also meanignless in English, so I can't really translate it. Das Bild ist vertig gemalt. = The picture is finished.