XF 2.2 Change (thumbnail/avatar preview) from circle to square


So i would like to change the preview of thumbnails from circle to square on every place of the site where we see them. (thread view page, search result page, tag result page and so on..)
Also, if possible, i would like to be able to change the size (wide and height). I guess for mobile i want a different size then for desktop.

I found nothing regarding this on the site here. I have no idea if this is even possible.
Anyone know if this is possible and how to do this best?
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