Hi,
You need to have your image in the folder of the "upload folder" option of the configuration or one of its sub folders and then, you can enter in your product the path from that upload folder for the image.
So by default, the upload folder is in media/com_hikashop/upload. So if you upload your image in media/com_hikashop/upload/my_folder/my_image.jpg, you need to enter in the product the path my_folder/my_image.jpg
In fact, you can use the "select" button next to the "add" button to simply select the image.