I've had a semblance of a responsive design on my sites for a while, but scaling images continued to elude me. I've always defined the height and width of images, meaning previous attempts at inline scaling resulted in some warped and stretched images.

With some help from Clara, I discovered Rubénerd specifically needed the following lines (your mileage may vary):

#content img {
  height:auto !important;
  max-height:auto !important;
  max-width:100%;
}

Now images, such as this Railgun image by Dream C Club, will preserve their aspect ratio while scaling up and down. Whether it will improve karaoke abilities is another question.