Too much noise already in the first one. Very nice composition though

First and foremost, you chose IS which is the premier (NUMERO UNO)
and the most difficult site to start your micro stock career.
Much like an IT graduate going straight to Microsoft for a job.
You could have tried the other sites which are not as stringent . ie. IS and FT last of all. Although I would never submit nature shots to FT ( unless you have something incredibly rare like Arctic or Grand Canyon, Whales , or Icebergs from the Maritimes,etc.)

Do not be so quick to give up on your intention to be a stock contributor. Many of us are also gallery and creative photographers who started without a clue about digital photography, post processing, etc.
But with time, all these mysteries became 2nd nature . My colleagues and I all started the same way. Noise? What? we only knew about grains, not noise, not banding, photo shop,etc.
But today, we could work on our images without even thinking about it. Chatting and half watching our favourite video,etc..

The gallery prints is very different . I agree. I belong to a co-op of exhibition photographers and yes, I see prints that sold for 500 dollars bearing all these "artifacts" and what not.   The buyers are quite oblivious to those things. Even when I pointed out those things, they at times look at me blankly and say, "oh !! .. this is going on the mantelpiece of our fireplace.  It matches the wall of our living room in our cottage"... or " I was one of the builders of that building".

Micro stock or any stock work is different. It's being used for publications,etc where the quality is vital.   How microstock photography has risen way past that level, as many have pointed out, "the bar has been raised so high, but the cash and commission has come down to almost kissing the dirt on the floor".
 There is no logic to that. But fortunately  some sites are slowly raising the commission to those who can produce the goods and quite happily so.

But these new techniques in post processing and control with your DSLR come in handy too in your work. At least me and my affliates know they do.
e.g. We don't always need to be retouching negatives and film media , now that we have also gained knowledge of the digital media.

I wouldn't suggest you quit and just give up. I'd say why not start as a "weekend" contributor. Most of us started that way too. In fact, 5 of my affliates and myself included, are still in that category ,as we don't have much time outside of our regular photography livelihood. But we are very close to retirement, and we like the idea of one day being able to just work at stock in our own time , as many successful stock photographers have. This takes time to build.

I cannot tell you how much we all have learned from being told about the problems that plague digital photography. There are simpler ways to avoid that.
As a photographer, you should be able to surpass those problems such as noise,
artifacts,etc.. with proper exposure and lighting.

It really isn't a mystery. If you shot in digital and have your exposure and lighting spot on, you have really very little post processing to do, other than the simple spotting or colour correction.
That will ensure your consistent approval at IStock.

P.S.
Invest in the best glass you can find. It's not expensive to buy  prime lenses that will do the job better than the more flamboyant and expensive zoom lenses.
If you prefer zoom lenses which I deplore, then make test to find the sweet spot
and stay with that. You really don't need every mm of a zoom lense to make sellable stock photographs. I use only 2 prime lenses to do the job, and you can get a far superior prime lense with the money you spend on any one of those fancy doodie mother of a zoom lenses. You don't really need those ;
not unless you want to be the dude with the biggest bazooka in the business 
Exceptions of course, if you're intending to specialize in sports . But you still need to know how to get the best from your equipment.  That being said, you already far ahead of say, someone else  who haven't a clue about photography .  So, really, it's a bit too soon to quit.

Sorry, I think I'm hitting quote instead of 'modify'.

Hey everybody, thanks so much for the critiques. We understand much better now why the photos were rejected and now we don't feel bad about it at all. (We were, naturally, a little stung, initially.) My wife and I, as I said, are nature photographers. Fine art is what we strive for and it actually does quite well in our gallery in the little tourist/artist community of Door County, Wisconsin. We're clearly not technicians or experts with the cameras or the editing software. We'd love to get better (and you all have helped us a great deal by pointing out what we need to improve as far as technicality) and we're going to work at it. But things like a bit of noise just don't concern us, though we understand why they concern you. We've never lost a sale because of noise. Our photography is just artistic expression; just trying to make something pretty that people will want to hang on their wall or send as a greeting to a friend.

Sounds like you've made the correct decision.