When I began with stock photography back in July 2011. I was using mostly kit lenses like my 18-55 and 75-300 kit lenses from Sony. I tought that this kit lenses will satisfy my needs in photography, since I was beginner at a time. Well to be honest those lenses was pretty good for me, until I begin to pixel-peep my images, or inspecting them at 100% magnification. As you all know, that is how DT reviewers inspect and aproove or reject our images, so you all now why is important that we do it also.
I have realized that kit lenses have some disadvantages over more expensive ones in stock photography. In my case I was having very much troubles with postprocessing in a way that I needed pretty much time to make my image to be acceptable for stock standards. There was a lot of work to remove or reduce highly visible cromatic abberations, which can be one of reasons for image rejection. Image sharpness was questionable sometimes, images sometimes ruined because of sun flare etc.
So I have decided that in near future I want to replace my kit lenses with more expensive ones. Firstly I have replaced my 18-55mm kit lens with a lot better 16-105mm, which has excellent image sharpness, have better image quality, better chromatic abberation and flare control comparing to my old 18-55 lens. I do not need to mention a way better focal range, I got wider angle and larger range. And this is also a great walk-around lens for my travel or vacation trips for example.
I did the same thing with my 75-300 zoom kit lens. I have replaced it with much expensive Tamron 70-300 USD, the closest thing to Sony 70-300G lens in image quality. G lenses are top in Sony lenses lineup and still to expensive for my budget. I have replaced this lens because I needed better sharpness and image quality in 200-300mm focal range for my butterfly, dragonfly, bird and animal photography.
Except better image quality and better chances for my images to be aprooved, I have also benefit from investing in better equpiment in a way that I have greatly reduced my image postprocessing time. It is because I do not have to fight with cromatic aberrations anymore, or flares or poor sharpness and similiar problems which comes with cheaper lens lineup. So I have saved myself a lot of time in my workflow with this upgrade. This is why I recommend your photo equipment upgrade.
Do not get me wrong. I am not saying that you will not be able to get a good and acceptable stock images with your kit lens. You can, but you will need a much more time in your image postprocessing. So better equipment will ensure that you spend more time behind your camera and shoot images, and less time behind your computer trying to correct a cheap(kit) lens faults on your images to get them aprooved and accepted on your microstock agencie.