As you can see by the screen shot I have all these textures that I used for doing some testing. And now I want to get rid of them. They also show up in the texture editor drop down box which I did not find a way to delete them there either.
Under the listing box in the Textures context is the texture name field and some buttons. The X on the far right unlinks the currently-selected texture. If you Shift+click (RMB) on this X, it with mark the texture for deletion when the file is saved and closed. You have to save & close the file for the textures to be removed.
Note that any materials still using these deleted textures will show an error upon re-opening the file, as Shift+X-button overrides all user links for that texture.