What other plugin are you using? try using the image store only to check for conflicts and if you still have problems change to a standard WordPress theme.
The “_key” is no longer used by the latest version of the plugin, if you have an older version -2.0 you will have to check file permissions.
One thing you may have to check is permission for the folder where you are trying to save the images.
If you still have problems, please check your server log and provide information on any error caused by the plugin.