Same problem in Maverick using the latest Adobe Flash for 64-bit.
This seems to be website-specific.
Try to upload an image to Flickr with Flash enabled - no preview
Try to upload an image to your Launchpad profile with Flash enabled - preview is OK.
It looks like websites can override the OS upload dialog with their own or something along those lines.
Same problem in Maverick using the latest Adobe Flash for 64-bit.
This seems to be website-specific.
Try to upload an image to Flickr with Flash enabled - no preview
Try to upload an image to your Launchpad profile with Flash enabled - preview is OK.
It looks like websites can override the OS upload dialog with their own or something along those lines.