Well I cannot look over your shoulder and have to take it as you describe your procedure.
So Sans Mp3 Art Sizer shows the new resolution in the row "New Size"?
And you are testing it really on files with embedded pictures? MP3Tag shows also your front.jpg in the folder if there is no embedded cover in the mp3. In this case it shows the name of the file too.
I ask this because you wrote:
So the thread started with the wish to resize your external files and then import the resized files in the mp3s and I am suspicious that you still have no embedded picture.
If this is not the case test this way:
Import a real large file in a mp3, maybe a digital foto from a camera and resize with Sans MP3 Art Sizer. I can tell you that the resizing is working because I just tested it again, used a picture with resolution 3488x2616, resized it with your settings and MP3Tag shows me the new resolution and a size of the picture, which is only 20 KB from former 4,13 MB.
Do you perhaps have mor than 1 picture embedded. I wrote somes posts above that Sans MP3 Art Sizer is only able to resize 1 picture in a mp3. It even does not show that there are more pictures if there are. If there are more pictures you can see in MP3tag.