Topic was moved from Photo.

Savio, it is possible via attachments. For instance, user can copy image URL from external sites and paste this image into blog post using the Insert Image icon.
In case you want users to use photo from external site on their Photo page, they will need to upload them to the site.  
So, to assist you more effectively we need to know what you want to achieve in more details.

i just want to allow my users to use profile pictures from an external website.

Savio, in this case users will need to upload the photo to the site. I mean that there is no any setting, for example in admin panel, that you can change to achieve needed results. So, to solve your issue you will need to develop your own plugin. If you are not familiar with the code modification or plugin development, it would be better for you to find a developer who can help you. You can do it here: http://www.oxwall.org/market/.