Uploaded images linked not previewed.
 

What causes some images to not display a thumbnail/preview and instead just giving a link?

I believe that happens when a .JPG file is slightly flawed. It doesn't mean that there is something horribly wrong with the image. All it means is that out software that makes thumbnails is not able to convert the image.

Thanks,

i have an other possible reason....
 

i have an other possible reason : when you save your photos in jpg with compression of under sampling chromizing starting up by this value YCbCr 1X1 1X1 1X1 !