CSS: Image-Scaling with Tumblr 'Astronaut' theme

I'm rather clueless with regards to CSS, and trying to use the free Tumblr theme 'Astronaut'.

Currently I'm trying to figure out how to scale images proportionally for the 'face' of the blog (which is in a three-row style) while keeping the image intact in the main posts itself. I've tried figuring out where to place the width:300px (though that's only a guess as to what the max width of each 'row' is) and height:auto tags in their block, but it doesn't seem to be working. This is exclusive to text posts, image posts work fine.

The site, for reference.

And a pastebin of the layout.

Thanks for any help you can give me.

Solution

Try again what you mentioned in your question by adding height: auto; into your CSS:

#container .box img {
    max-width: 100%;
    height: auto; /* add this! */
}

This should scale the image down to the available width of the parent element if the image is larger and keeps the correct proportions of your image.