How does one "manage attachments to free up space?"

[applcin]

Didn't know quite where to ask this but I seem to have reached my quota size of attachments. It tells me to "manage attachments to free up space." I would like to delete some photos I used in older posts. I noticed just deleting the images from the posts they were in doesn't do it. I can still see them on the My Attachments page. What I don't see is a delete button next to any of them. I'm using Chrome if that matters.