I have Photo Stream set up and my pictures show up in iPhoto on my Mac. Do I have to import them to keep them permanently? How can I free up iCloud space?
Open the iCloud preference pane on your Mac to get the easiest way I've found to manage the storage.
Click the Manage... button in the lower right.
This will calculate all the storage users counting against your quota and offer you details on how to clean up each category. Once you've reviewed each bin - you can decide to delete or if you would rather purchase more storage to keep what you have stored in the cloud.
My experience is people can do one step to free up significant backup space from iOS. Go to the Settings app and then iCloud > Storage & Backup > Manage Storage
Select the device you are on and then under BACKUP OPTIONS - turn off Camera Roll and possibly any other items you don't want backed up.
This means you need another way to back up your photos, but since you have a Mac - you could set iPhoto to automatically back up all photo stream contents or regularly sync the photos and videos from iOS to the Mac and then back up the Mac to a cheaper hard drive or service than iCloud.
Photo Stream does not consume space in your iCloud account so deleting photos from Photo Stream will have no benefit. To reclaim space, delete data from other services such as Documents and Data, Mail, and iCloud backup.