Has to be technically perfect to pass submission. (for alamy at least)
Has to be something that people will use or want to use.
Has to be be different from the 100000000000 other stock images
Has to be free from copyrighted logos, items etc etc unless for editorial use.
Think of all the photos you see in books, magazines, adverts. Chances are they are from stock. So take images like that.
I submit to Alamy and Getty.
Sold 2 on Alamy in about a year. Both of animals, one of a chicken, and the latest last week of my ducks.
I got $70 for the last one, a print run of 10,000 copies for a book. After commission, I get about £20.
If you think it's an easy way to make money, think again.