DRUPAL 7: insert images via URL instead of via "file-upload"
I guess you are having problem to use the module File Field Sources. 

Use below steps


Download and install the module 
Edit any image field, for ex. Article content type is having image field. Let's use this module over there. 
Go to URL : admin/structure/types/manage/article/fields
Edit the field field_image 
Check the option to use remote URL there. See screenshot. 
Create article content and see there option to use remote URL in image field. See attachment.