My point is that the html editor has not altered the image, it cannot. It can put in markup that tells the browser to display a large image in a small way, but it does not alter the image itself.
The danger here is that people will post big images (big in dimensions and big in file size) and simply use the 'handles' in the editor to resize the image. Everything displays fine, but behind the scenes, an image that only needs to be, say 200x200, and 10kb in size is actually 600x600 and 700kb in size.
If you have a visitor to your site on a dialup, and they display the first 10 entries to the glossary, and this is happening with even one or two entries, the page will be slow to load.
I think that if you want to display a 200x200 version of an image, then you should (must, from an accessibility and usability point of view) edit and save a version of your image with these dimensions and quality. Here is exactly the same image, being called from the same place, but displaying in 2 different ways. I resized the second one by dragging the handles, but I did not alter the image, only the instructions I give to the browser about displaying it: