If you want to delete the storage account, you need to delete any images/disks you have remaining in that storage account.
Go to Virtual Machines -> "Disks" or "Images" tab -> select the images/disks that you have located in the Storage Account you want to delete -> then hit the "Delete disk" or "delete image" command. Back these up if you need before
hitting the delete.
Once the images/disks in the storage account have been deleted, you should be able to delete the storage account. Hope this helps.