Stock photos... I have about 20 images on Getty, I'll offer my two cents:
First off JeffW is correct on pay outs. Most stock sites that have, let's say, lower quality work will more readily accept your submissions, but the pay outs are pennies.
It's a lot of work to get them ready. Some sites want full size TIFFs, some want certain size JPEGS. If you're uploading to many sites, that's a few hours behind the PC you got to be willing to put in.
Now, the non fun part. To actually make money, you'll have to submit your work to the big sites like Getty. You submit your best images, and wait to get approved. After approval, you still have to have your images you wish to sell approved before they go on the 'for sale' block.
Getty ( and the few other top stock sites ), the payouts are good, the images are very specific, and they protect your work by only allowing a buying customer to purchase, no free reign for xxx a month to buy xxx of images. A lot of the 'common' stock sites do this.
Your submission shots, should not be your 'stock images'. Showcase yourself to the stock agency. Show them your actual talent, this shows you are actually skilled and can possibly make them money. This helps you. If you sell a few, you may get a certain request for a certain stock image to take and then provide for sale, not common, but it happens.
If you're going for the bigger sites the submission processes... I'm not going to lie ( and I'm not saying this to sound arrogant ) you HAVE to have pristine work. And I mean, pristine in the most sincere way possible. If you do a lot of depth of field ( they sell well for flowers / birds / stagnant objects ) you background has to be nothing but a smear of bokeh. Composition and exposure need to be 100% perfect. On Getty, the buyers are paying good money PER image, so Getty requires top notch work only.
Go have a look at Getty Images, see what's out there, see what kind of quality you're aiming for. Get some ideas for stock imagery you have an interest in and practice every chance you get.
Couple more hints :
Landscapes: You gotta be Helen Dixon or Lee Frost good, it's that simple. If you don't know either of those two, look them up.
Birds / animals : extreme DOF must be the core. Eyes sharp as tacks. No misplaced feathers, hairs, blood on face, etc. Background must not be recognizable
Stagnant, standard 'stock images' : yours has to stand out from the rest. Everyone can take a nice image of a cup of coffee on a kitchen table. How can you take it so your image stands out from others?
Office / General People / Task specific : one image should convey a description. Like, guy answering office phone / woman showing stress in front of PC. Random people photos are a no. Street style, no.
Architecture Images / Art : never seen one sell, tbh. Who would buy a picture of a painting? Most buildings that require a picture OF their building, simply hire out a photog. No company would buy a stock image of their own company.
Sorry about the book lol. Just trying to give you everything I got. You can make good money doing stock images, but you've got to be an excellent, excellent photographer in more than one genre. If all you do is birds, you're limiting yourself. If you have the time, you can do 100 stagnant images ( office supplies, drinks in glasses, fruit trays, food ) a week and upload them all, better your chances. Good luck
EDIT: I should convey I don't sell 100 images a year on Getty either lol. I've made quite a few sales, nothing crazy. Don't want it to come across like I'm a high and mighty ego driving photog... nowhere near
