Oh, so like the concept in that Magicka game? I'd say...instead of just having the spell become stronger in relation to how good the drawing is, have it so that the better the drawing, the more mana-effecient the spell will be. So she has the option of putting in lots of mana in that super-effecient spell to get out a big effect, BUT can also choose to use a neligible amount of mana to get out a decent effect.

The drawings should disappear after they've been used once though, so she can't do something like spamming the same spell over and over again. It'll make her more like a tactical nuke than a missile shower, though.