I think the image will need to have a square aspect ratio in order for the rounded-border result to be a circle. Thus, it will either need to be cropped, or if you can somehow position it within a square container and then round the borders on the container, it might work. I haven’t tried the latter, so you might have to tinker.
You could do the following, but it’s not going to behave the way you want / expect. The group will not maintain aspect ratio for different viewport widths.
Imgix would be great for this; it can even recognize faces and crop so that the face is centered. However, Bubble doesn’t officially support Imgix URL tinkering with its native integration, so you’d be better off getting your own Imgix account.
But I forgot that there is a built-in circle and face cropping option. Check out :processed with Imgix in the expression builder. Definitely worth experimenting with that first.