It appears that there are two things happening here. The first is that my quick check of the code suggests that if GD wasn’t properly installed and configured when the image was uploaded that you won’t see the image-size options enabled. This comes from looking at wp-admin/includes/media.php
function image_size_input_fields($post, $checked='') {
// get a list of the actual pixel dimensions of each possible intermediate version of this image
$size_names = array('thumbnail' => __('Thumbnail'), 'medium' => __('Medium'), 'large' => __('Large'), 'full' => __('Full size'));
foreach ( $size_names as $size => $name) {
$downsize = image_downsize($post->ID, $size);
// is this size selectable?
$enabled = ( $downsize[3] || 'full' == $size );'
appears to be the function that determines if the buttons are enabled.
A definition of image_downsize() appears in wp-includes/media.php
/**
* Scale an image to fit a particular size (such as 'thumb' or 'medium').
*
* Array with image url, width, height, and whether is intermediate size, in
* that order is returned on success is returned. $is_intermediate is true if
* $url is a resized image, false if it is the original.
*
* The URL might be the original image, or it might be a resized version. This
* function won't create a new resized copy, it will just return an already
* resized one if it exists.
*
* A plugin may use the 'image_downsize' filter to hook into and offer image
* resizing services for images. The hook must return an array with the same
* elements that are returned in the function. The first element being the URL
* to the new image that was resized.
*
* @since 2.5.0
* @uses apply_filters() Calls 'image_downsize' on $id and $size to provide
* resize services.
*
* @param int $id Attachment ID for image.
* @param string $size Optional, default is 'medium'. Size of image, can be 'thumbnail'.
* @return bool|array False on failure, array on success.
*/
function image_downsize($id, $size = 'medium') {
which suggest to me that thumbnail (or other sizes) aren’t likely to “magically” appear when GD is installed and things are restarted.
The second thing is that the wordpress.com behavior of https://blogname.files.wordpress.com/year/month/filename.jpg?w=128&h=96 does not seem to be a “stock” feature of either wordpress or wordpress-mu.
