Multiple instances of the same image all get resized to the width/height of the first image

This happens because you're storing just the URL/path to the original image instead of the entire img tag in image_resize_filter_get_images():

  preg_match_all('/<img[^>]*?src[ ]*=[ ]*"([^"]+)"[^>]*>/', $text, $matches);

To support multiple instances of the same image in the same textarea with different sizes, you must store the entire img tag.

I know, very minor, but in my tests with resizing images, I was puzzled why this wouldn't work.