How to add parameter after image url?

[fingon]

Hello.
I want to know how to add parameter to attachment images, for example:
this is normal url
http://www.mysite.com/data/attachment-files/2015/12/11_a21f979cfdba18bccdd29e536058f68d.png
I want to add parameter after the url ,like this:
http://www.mysite.com/data/attachment-files/2015/12/11_a21f979cfdba18bccdd29e536058f68d.png/PARA

I have an idea that do nothing in precess to write data into database, and add /PARA when reading image url from database.

But I dont know which php file I need to edit ,any tips?

thanks

 

[fingon]

ehmm, maybe in templates ,trying to find a way to do it