Ricky has provided links to useful Help pages which are full of info to assist you in becoming in becoming successful as a Stock Contributor.
This page specifically addresses the do's and don'ts of image processing, and makes the following points:
https://helpx.adobe.com/stock/contributor/help/photography-illustrations.html
Image adjustments
Do: Straighten horizon lines where appropriate.
Do: Adjust levels, curves, or brightness/contrast to ensure the best tonal range.
Do: Reduce noise if necessary — but don't overdo it, as you could create a soft image.
Do: Get to know sharpen tools and use them carefully — too much sharpening can bring out noise and unwanted texture.
Don’t: Aggressively lighten shadows as this can introduce noise.
Don’t: Convert your image to black and white or duotone. Buyers want the maximum flexibility provided by the full color image.
Grain/noise may be something you like seeing in your photos for your own use, but makes them unsuitable for Adobe Stock. It is not something that the Buyers want to deal with. If they want a grainy look, they'll add it themselves.
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