Disk Utility showing unfamiliar Disk Images

When I go to Disk Utility, I'm presented with a tree on the left with disks mounted to my machine, I'm not sure I understand where the 'Disk Images' branch came from. It is worth knowing that I've installed VMware, Virtualbox, Xcode and a lot of other software because I was trying to run a linux vm.

When I try to eject the disks I get the message:

"Failed to eject “Apple Disk Image Media” because it is currently in use."

How do I remove these disks and regain the used space?

[Edited by Moderator]