The fact that there is a picture similar to one you would like to submit is not sufficient. If they have one and they decide is sufficient for their market, yours is not needed (it's not you their aim ).
Months ago I wanted to try, but I did not at the end, because too boring. However, it could be a great exercise for a beginner to shoot some picture specifically for those sites - it is not matter of using what available, but shooting what they need. E.g., as far as I understood, working people, but with model releases.
There is next to no money to be made in stock photography. Unless you have your finger on the pulse of exactly what these stock photo companies need and have the time and talent to produce thousands of usable quality images. My humble opinion, your time is better spent elsewhere searching for money, if money is your ultimate goal.