I will take a stab here-

1. Commercial value- think what value the photo has before shooting it
2. ISO 100 - make sure there isn't any noise whatsoever
3. Study the most popular images on Shutter to see what is selling and see if you compete with them - notice I said not copy them. See if you can produce high level images

About the antenna I agree that it is a pity that it is cut.
But with some postproduction I think that you can solve this little problem easily.

About the space at the bottom, you should consider that it is better to have too much space than too much little because the space can be cut then by the designer.
You should consider too than an empty space with an enough neutral tonality can be used to put text on it, or insert other images or graphics.
So, from a designer point of view the space at the bottom is absolutely not a problem but a solution

Because I also would like to start making residual income with something I'm passionate about!

Because I also would like to start making residual income with something I'm passionate about!

"Residual Income" might cover your expenses for a soup & sandwich once a week.  I consider myself a major rookie and can tell you don't expect to see the dollar/pounds to come flying in unless you are willing to put in long days and learn the business very fast. The competition is very high to say the least thus most rookies try a few months and quit. Keep your expectations realistic especially the first few years....

I don't know about iStock. But I recommend you try Shutterstock. Go to their forum, post 10 photos and ask for their feedbacks. You will learn tremendously by trying to get into Shutterstock even if you have no intention to contribute there. Once you are accepted by Shutterstock, you are ready for most microstock sites out there include iStock.

Solid advice!