As for an ad management plugin, I'm using WordPress' default media uploader. And WP Media Uploader automatically resizes images. But as the ad image is already resized (i.e.: 225 x 100px) it need not to resize into a new file for thumbnail (100 x 100px); because, that thumbnail is unnecessary for the site.
The cause I presented here, may be a very minute for such a solution, but in case, there is any other massive issue that, in such plugin we can stop the auto resizing with WP Media Uploader only on that plugin-uploads - I think that can be a good solution for any site.
Is there a way to use WP Media Uploader, but can stop auto resizing images only onto that particular case uploads?
But I also know that, plugin like Regenerate Thumbnails can override the rules and can do what I prohibited in my plugins for the site's space-savvy consumption.