I always try to squash as much of the complexity as I can with some well tested tools or libraries. In this case I'd use imagemagick's convert to resize and con

I always try to squash as much of the complexity as I can with some well tested tools or libraries. In this case I'd use imagemagick's convert to resize and convert any of the possible file formats to the type of image I need. Solves a lot of points on this list. For the download part, curl would also solve a few points (ssl, redirects). Of course it depends if you're writing this for a SaaS product, a desktop or mobile Application.