the association would generally be for if you had a mail client installed on the system. Since gmail is web based, you do this a bit different.
You login to gmail(in your web browser) then start to compose a new email and use the attach a file(or paperclip icon on the new compose window) to upload the image to gmail for sending.
Compose and select Attach
Once file(s) have been selected, they will show up at the bottom of the e-mail. You can now send them.