I have no idea what the Dreamwidth image proxy for pictures hosted elsewhere does to fetched image sizes, sorry. (And as it's written in a programming language I don't understand, it's unlikely I ever will unless someone tells me.) You may have better control by uploading those images to your Dreamwidth account and editing your entry to use the full-size image URLs with the width= attribute. (Or maybe not because of https://dw-dev.dreamwidth.org/217192.html?thread=1689448#cmt1689448 . CSS and JS make my head hurt.) See https://www.dreamwidth.org/support/faqbrowse?faqid=248 .