This image ended up being pretty crazy. I went through a bunch of variations trying to fit a furnace into a man's chest, and then trying to peel his torso open and inserting a funnel linto it in which a load of things are being poured. I also thought of imagining a black hole absorbing things into his heart but the difficulty that I found was limiting what I intended to say visually and also making it work as a visual piece. So I eventually stumbled across the idea of using vacuums for sucking things up into the heart. It's kind of creepy but it works.
Commentators had a few similar things to say about the meaning of this proverb. One of the biggest things is the significance of the division between our ability to make or intend something and what actually 'is'. Despite our greatest efforts, and often wasted efforts, what will be will be not just because of or solely dependent upon our efforts.
Many commentators used the example of the Sorcerer, Balaam, who was paid to and attempted to curse the Israelites but was restricted by God in what he was able to say so that he could only bless them. Despite his preparations (and payments) towards hurting them, he simply couldn't.
And another commentator pointed out with simplicity that this proverb demonstrates how dependent we are upon God for even the simplest act of speaking. The big question that pops out after that is of course why God has people say so many of the horrible things that we say. I don't know. I think the answer is in Job.