Nice work, now we need to bundle this into a plugin.

Instead of changing a few files like my last post, you just need to change one file and that is wp-includes/media.php

row 434
old code

  // $suffix will be appended to the destination filename, just before the extension
  if ( !$suffix )
    $suffix = "{$dst_w}x{$dst_h}";

new code

  // $suffix will be appended to the destination filename, just before the extension
  if ( !$suffix ){
    //thumbnail
    $max_width = intval(get_option('thumbnail_size_w'));
    $max_height = intval(get_option('thumbnail_size_h'));
    
    if($dst_w == $max_width || $dst_h == $max_height)
      $suffix = 'thumbnail';
    
    //medium
    $max_width = intval(get_option('medium_size_w'));
    $max_height = intval(get_option('medium_size_h'));
    
    if($dst_w == $max_width || $dst_h == $max_height)
      $suffix = 'medium';
    
    //large
    $max_width = intval(get_option('large_size_w'));
    $max_height = intval(get_option('large_size_h'));
    
    if($dst_w == $max_width || $dst_h == $max_height)
      $suffix = 'large';
    
  }

This code will check with the values you put on the various sizes and if there is a hit, the suffix will change accordingly. I haven’t tested it so much so if someone sees a mistake somewhere or can apply with a better solution, please do.

I think I’ve managed to solve the isseu where you want the naming to be image-thumbnail.jpg, image-medium.jpg and image-large.jpg. If someone else have another good idea, please do tell.

The following was done with some core files (wordpress 3.1.1) :

in file wp-admin/includes/image.php

row 107
old code:

$sizes[$s] = array( 'width' => '', 'height' => '', 'crop' => FALSE );

$sizes[$s] = array( 'width' => '', 'height' => '', 'crop' => FALSE, 'image_size' => $s );

row 125
old code

$resized = image_make_intermediate_size( $file, $size_data['width'], $size_data['height'], $size_data['crop'] );

$resized = image_make_intermediate_size( $file, $size_data['width'], $size_data['height'], $size_data['crop'], $size_data['image_size'] );

in file wp-includes/media.php

row 406
old code

function image_resize( $file, $max_w, $max_h, $crop = false, $suffix = null, $dest_path = null, $jpeg_quality = 90 ) {

function image_resize( $file, $max_w, $max_h, $crop = false, $suffix = null, $dest_path = null, $jpeg_quality = 90, $image_size = null ) {

row 434-435
old code

  if ( !$suffix )
    $suffix = "{$dst_w}x{$dst_h}";

  if ( !$suffix ){
    if($image_size != null)
      $suffix = $image_size;
    else
      $suffix = "{$dst_w}x{$dst_h}";
  }

row 484 (or maybe higher because of alteration of code above)
old code

function image_make_intermediate_size($file, $width, $height, $crop=false) {

function image_make_intermediate_size($file, $width, $height, $crop=false, $image_size = null) {

row 486
old code

$resized_file = image_resize($file, $width, $height, $crop);

$resized_file = image_resize($file, $width, $height, $crop, $image_size);

Good point. This scenario doesn’t take into consideration alternate image sizes. We will look into this.