As a starting point, see popular subjects in your Contributor User Guide. 

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-submission-overview.html

Well executed images of smiling business people doing things or in meetings are - oddly enough - still in high demand. Otherwise, it is difficult to predict. What you think will sell doesn't sell, and vice versa. Luck plays a major role.

I usually think a mix of both approaches works best. Looking at search demand, current trends, and how crowded a category is can help identify opportunities, but creating something you actually understand or enjoy can make it easier to produce better work consistently. I’d also check what is already performing well and look for smaller gaps rather than chasing every trend.

I don’t  do much “market analysis”. I acquire  images from my travels around the world, and upload the most interesting and highest quality from those. These types of images will remain evergreen for years to come, and are not subject to whatever current whims and fads are gripping the markets…

I don’t think about it. Trends are as fickle as the wind. Here today, gone tomorrow. I think it’s better to draw inspiration from art movements that have made a lasting impact on people. And then incorporate those concepts into our own artwork.

https://www.invaluable.com/blog/art-movements-painters-palettes/

Customer buying habits are hard to predict. Nobody expected that ‘people in surgical masks‘ would become a huge seller until COVID hit. 😷🤒

Life just happens. Our job is to represent it the best way we can. If there’s one key to reaching the widest possible customer base, it’s creating a large portfolio that’s rich with diversity. 

Make lists of things that interest you. Compare your list with available Stock inventory to see if it’s something that Stock is in short supply of. When you find a need, fill it. Rinse & repeat.

Hope that helps. 

I just keep in touch  with the rest of the needs that we have as graphic designers specially with subjects in wich we curate stock content for clients and we find lack of content.

Market trends are oversaturated. Important is to continually supply high quality assets that are in demand. This helps keep your assets at the top of searches where they can be seen.