The image style used on the ckeditor5 dialog form is hard coded to thumbnail. This isn't ideal as some sites may have this style overridden or may wish to use another style.
Also the image alternative isn't using any image style.
Create a template and upload a preview image in the Illustrative Image/Icon field, then inspect the markup of the icon in the dialog form, it's always using the thumbnail image style url.
Expose the image style in config. Regarding the alternative images, they should probably be downloaded first before using them in the dialog, that way an image style can be applied consistently, which isn't possible now.
Needs review
1.0
Code
The image style is exposed to config with the code from the MR. The part with the download of the alternative icons is yet missing, but I think this can be reviewed as is and the rest added later.