Received first rejection on first batch of eight.

Machine generated AI images are inherently flawed with mistakes.  And AI isn't smart enough to know what mistakes it makes.  It's the contributors job to carefully inspect every quadrant of every image at 100-300% magnification.  Remove unwanted artifacts and fix the errors if you can.  Otherwise, discard the image from your submission stack. Rinse & repeat.

Compare 5-10 of your very best images with current Stock inventory.  Is yours as good or better than what Stock has?  If not, don't submit.  That should help reduce your rejection rate.

In my experience with Midjourney, it takes 100 or more diffusion cycles to get 5 decent images to work with.  There are additional fees for high resolution upscalers and you MUST have a pro level membership plan that contains "Stealth Mode."  Stealth Mode allows you to move images & prompts out of public view until you're ready to publish them.  Without this safeguard, you're at the mercy of anyone on Midjourney who wants to use your AI artwork/prompts without your permission. 

Hope that helps.  Good luck.

And several others were rejected. I can't actually understand why?
If anyone can shed any light so I know how to improve for future uploads.
I use Topaz Gigapixel Ai

By @Rob Art Illustration

Others use Topaz too, and it looks that the tool works fine. But you always need to check your assets before upscaling. Any defect in the image gets upscaled too, and upscaled, it is much easer to spot.

There was a precise reason: Quality issues, meaning that something in the image is not correct. Post the image  in full size here.

Use the drag and drop area:

If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

First of all there is a focus issue on the nails, however more interesting is six fingers and a thumb on his hand.

It is not possible for us to evaluate the quality of this image based on the tiny inset image you have provided. Upload here at full size if you wish to receive valid feedback.