Featured image does not scale at small screen sizes

I think this might solve it. There may be a more elegant solution, but I just stumbled upon this one.
Add the following lines to your style.css file.

@media screen and (min-width:761px){figure,.wp-post-image {max-width:710px;}}
@media screen and (min-width:621px) and (max-width:760px){figure,.wp-post-image {max-width:600px;}}
@media screen and (min-width:480px) and (max-width:620px){figure,.wp-post-image {max-width:450px;}}
@media screen and (max-width:479px){figure,.wp-post-image {max-width:260px;}}