Hi,
in file /administrator/components/com_jsn/helpers/fields/image.php you will find the code to crop and resize the images.

if you not use "crop & webcam" function the code is from line 138 to line 154
if you use "crop & webcam" function the code is from line 246 to line 254 and from line 290 to line 300

to edit images we use Upload Class libraries, you can find docs to add watermark at http://www.verot.net/php_class_upload_docs.htm