Width-only scale (and preserve aspect ratio)

And here we go, the solution. No time to produce a patch atm.
In inc/imce.page.inc, function imce_resize_image()

<?php
function imce_resize_image($filename, &$imce, $width, $height, $copy = TRUE, $dest = FALSE, $op = 'resize') {
  $dirpath = file_directory_path() . ($imce['dir'] == '.' ? '' : '/'. $imce['dir']);
  $filepath = $dirpath .'/'. $filename;

  //check if the file is an image
  if (!$imce['files'][$filename]['width'] || !$img = imce_image_info($filepath)) {
    drupal_set_message(t('%filename is not an image.', array('%filename' => utf8_encode($filename))), 'error', FALSE);
    return FALSE;
  }
  
  // new stuff begins here.
  
  if (!$height && $width) {
    // auto-adjust height to fit aspect ratio, if no explicit height is given or it is set to zero.
    $height = round($width * $img['height'] / $img['width']);
  }
  elseif (!$width && $height) {
    // auto-adjust width to fit aspect ratio, if no explicit width is given or it is set to zero.
    $width = round($height * $img['width'] / $img['height']);
  }
  else if (!$width && !$height) {
    // neither width nor height given - we have a problem.
    // TODO: Show an error message, but don't break the ajax request.
  }
  ...
?>

EDIT: Changed the code a bit.

I would say, just commit the above code, and later think about better documentation on the configuration form.

The current behavior for "120x0" or "0x120" or "120x" results in an ajax error. So, I think it is a good idea to do something meaningful instead.