I has cleaned up my media items by removing all the medium and large size from upload dir. So the medium size now could not be found.
Currently the Media library popup (when click Add media) uses medium for default and because of my medium size is lost, it uses full size instead, and it causes a slow load time. How can I change the display size of preview image in Media Library from medium to thumbnail.
Related:
How to change the image size in new Media Uploader (ie use medium vs thumbnail)
From fire bug, you can check where the image is rendered and then you can do it by simple styling.
sample_style.css