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Whoa -- I just re-read the text of minor illusion. When you use that spell to make a sound, it can be any sound you (the player) choose. But if you use it to create an image, it DOESN'T say that the player chooses the image! Surely this must mean that the DM can choose the image. Although of course the player can express a preference of what image is created.
Holy crap, it looks like silent image has the same wording -- you can alter the image, but it doesn't explicitly say that you get to pick it, so it must be the DM's prerogative. It looks like phantasmal force is safe, though, since it says you create a phenomenon "of your choice."
Also, polymorph transforms a creature into a new form, but doesn't say who chooses the new form, so presumably this means the DM selects the form. And even message is ambiguous because it says you speak a message but doesn't say that you get to pick what the message is, so it is up to the DM.
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In case it's not yet clear, I think the ruling on "conjure" spells is a little bit bogus. In general the caster of a spell should get to pick all of its parameters unless the spell explicitly says otherwise. Plane shift is a great example of a spell that explicitly says otherwise -- the player gives a general destination and the DM has broad latitude in where the PCs actually wind up.