Near the bottom of the left hand side of you Settings page:

There is a link (in the Miscellaneous) titled attachments this will take you to the following page:

In this area you can view attachments you have added, which threads they are linked to and also have the option to delete them and therefore free up space.

To delete an attachment put a tick in the checkbox agaisnt it (right hand side) then scroll to the bottom and hit the delete button