Generally speaking, uploading screenshots is very straightforward, but the extension icon upload is a problem. The system doesn't make it easy because it has to be sized so differently than the screenshots. We're working with our product team to make this easier for everyone.
For your account in particular, the image looks slightly better now. You appear to have chosen to include graphics with smaller elements (the heads of the people for instance) which can make the icon not look as clear. Also, you added company branding to the upper right corner, but it is so small that you can’t easily read it. The issue might not so much be an image sizing issue, but instead a design issue - if you're able to change some design aspects, that should help you out here.
