Thumbnails Settings not Displayed

Hello everybody here,

I’m trying to scale images within the media library, for a faster loading time of this page, but once they are scaled, the thumbnails settings are not shown anymore (when I insert the images back into the page). And if I restore the original size of the scaled images it stays the same : no more thumbnails settings (I mean the choice between thumbnails, medium, and large) the only choice left, is to insert the original size then. I tried many different sizes when scaling, big ones, very small ones. The problem stays the same.
I’ve checked the media settings, the shadowbox settings, the W3 total cache settings. And of course I empty all the caches every time. Nothing helps. Something to do with SmushIt maybe?
I hope I’ll not be obliged to reload all my images!
Any idea?
Thanks a lot