Answer 1

Easiest way would be to scrape the webpage.
Simply download the html (with wget e.g,...) and find files ending with the extention. Something that ends with jpg is most likely jpg, png, svg, etc
Is there a more specific use case you have? If you simply want a picture in a specific format do a local conversion (like jpg -> png) using a library like imagemagick.
But, You cannot download something in any format you want which the server doesn't serve.