the portfolio preview is the same (~425x320px) on every profile to make it look consistant.
If you use a pictures with those measurement or something with a similar aspect ratio then you need have to crop it.
You always have to crop pictures with a different aspect ratio.
I hope I was able to help you.
Thank you, Lucas.
In my opinion, automatic image cropping is too strong but I understood that freelancer may to upload thumbnails directly and adjust a cropping manually.
You may simply need to use trial-and-error to get your portfolio thumbnails to show up the way you want them to. Upwork's system for uploading has limitations. But I have found I can try a few variations and get things to show up acceptably, while working within the limitations and actual functionality of the system.