@Franco_N: In your theme's CSS file, you have this:
p img {
padding: 0;
max-width: 100%;
}
That "max-width" limits the images to the width of the column, which is 450 px in your case. However, you don't have anything to fix the height. If you add height: auto; to that same block, the height will be scaled properly as well.
Like this:
p img {
padding: 0;
max-width: 100%;
height: auto;
}
However, that may not work in IE, only in Firefox.
For that particular theme, you should have $GLOBALS['content_width'] = 450; in your theme's functions.php, so as to limit the size of the images to the width of the column (which is 450px). This will only take effect on new image uploads, not ones you've already uploaded and inserted into posts previously. This fix WILL work in all browsers, since it's server side.