WARNING: This addon is very old and I no longer have time to support it. Use at your own risk.
Imgur-Screenshots
Put the file in lua/autorun/client
Go to https://api.imgur.com/oauth2/addclient to create a client-id for your imgur account. Fill out a name/description for the addon, and put your email address. Take the client-id they give you and enter it into the lua file where the comment is.