Firefly Generative AI illustrations for Adobe Stock

All Firefly aspect ratios meet the 4 megapixel requirement, and there is no requirement to upscale.

Firefly is good with some prompts and bad with others, so it's really a case of trying out different prompts, styles, techniques etc. to find something you can work with. I don't think the campervan is a good one to work with because it has too many artifacts.

For Stock images I like to use Firefly to generate the main subject of my image, then I bring it into Photoshop and make the canvas size quite a bit bigger and I use Generative Fill with a blank prompt to expand the background around my Firefly image. I do this in multiple 1024x1024 blocks because that is the maximum size that you can generate with Generative Fill - anything bigger is just 1024x1024 upscaled. When I've filled in my canvas I flatten the image and add a Levels Adjustment Layer and use that to crush the white level, which helps show up where joins between the 1024x1024 blocks are noticeable - usually in the sky or in gradients. Then I fix those joins up with the color mixer brush and remove the adjustment layer. Finally I downsize the image to something closer to the required 4 megapixels and save as a jpg.

Several flowers on the van are poorly drawn. 

The driver looks like a cat.  Is that intentional?

Nice image, but beside the awful wheels, you have other artefacts like this one: 

Those issues and the like need to be addressed, before you can submit.

I don't see anything in this rejected image that would indicate you are not using Firefly correctly. It just needs some editing. The smallest bird could be removed, the wheels and spokes aren't rendered as well as the rest of the image, and there's a white thing that I assume was meant to be exhaust at the rear of the van that could be removed. 

You could join the Adobe Firefly Discord channel. They often do livestreams .