trinorthlighting,
Please take this post as the constructive feedback for which it is intended.
While on the surface it might sound like it's as simple as pointing things to a different URL, IH completely changes the ZC image handling.
So what you're asking for is not a ZC core code change. It's an IH code change.
So ... ya, if what you really want is to change its functionality, feel free to change the PHP code to do that. You'll lose all the sanity checking that it does about the image file actually existing, unless you build in a check to have it actually look at the CDN server and verify the presence of the required image file (which adds increased delays on every check for every image), you'll also end up with broken image links on your pages anytime you make an image adjustment.
So then there's the issue of keeping the CDN up-to-date with new images added. While on the surface you may think it's as easy as periodically uploading changes, you'll have to code for it to handle when an image is missing, as hinted at in my last paragraph.
Your post implies that whatever CDN you've chosen apparently does all this self-updating automatically. That's not the norm. And a proper solution should be more sophisticated, such as actually intentionally pushing changes to the CDN when changes are made, not forcing it to "discover" during failures, which results in poor end-user experience.
Those are just the beginning of the changes. Can it be done? Sure! Are there LOTS of things to consider from LOTS of angles? Yup!
So, back to my first comment: on the surface one might think it's as easy as changing URLs. Feel free to start there. Everyone here is telling you that it's much more complex than that, and they know that there's no point in telling you only how to do that, because it will just break things on you. Nobody's trying to be rude or disrespectful. Just realistic.
You need to hire someone who's willing to take the time to study the implications of your proposed changes and then code things to work as you desire.
Then there's the issue of "why" you're even looking at this in the first place. There are some poorly-tuned servers that simply can't serve images well, and in those cases a CDN might help, but it's really a bandage.
There are also those in the camp of "use a cookieless domain to serve images because it'll improve speed by XY%", and even Google used to suggest this, but have relaxed their recommendations for this with the improvements in modern-day servers which can serve content just fine from the same domain. So people following this path based on that reasoning are largely following aging seo advice. Not that it's obsolete, but it doesn't pack the punch it used to. It's actually slower to load images from multiple domains because of the latency required to look up the address of those other domains.
And, yup, there are two opposing camps on these so-called seo-tweaks. Half of them are right. Half of them are in both camps. Half of them use crap servers so are looking at the wrong problem in the first place.
I'm not saying what you're attempting is wrong or pointless. I'm saying to make sure you're prepared to do it correctly if you do it, and to review it again and again every 3 months to be 1000% sure it's actually giving you something good, and that you're testing it on as many relevant browsers as possible. Because it might be wasted effort.
Just don't follow the flavor-of-the-month because it's popular or because some self-proclaimed guru said it's the best thing since sliced break. Make sure you have empirical evidence to guarantee the value of what you're about to do. This applies to everything, not just the subject at hand.
As far as doing what you started posting about, look at the IH code (you can see exactly which files are involved by looking at the plugin zip itself), and go from there. Little point in stepping out of that into ZC core since IH intercepts that almost exclusively.
