I tried your plugin on a beta version of WordPress 3.6-beta4-24560. and when I tried to set the thumbnail image after I uploaded it. It gave me the message below.
Notice: Undefined index: alignment in *** test.robertdall.com/wp-content/plugins/easy-watermark/lib/EasyWatermarkSettings.php on line 156
Warning: Cannot modify header information - headers already sent by (output started at *** test.robertdall.com/wp-content/plugins/easy-watermark/lib/EasyWatermarkSettings.php:156) in /home/robertda/public_html/test.robertdall.com/wp-includes/pluggable.php on line 875
I know I am using a beta version of WordPress. But since were very nearing the release of 3.6 I would mention this.