Claim an Imgur album that I made before I signed up

From the TOS, this would seem to suggest that nothing happens to your licensing.

Imgur Terms of Service are subject to change without notice. All files are copyrighted © to their respective owners. Imgur directs full legal responsibility of files to their respective users.

To answer the other part of your question, AFAIK, there is now way to associate any particularly licensing information to an image hosted on imgur. IANAL, but it sure seems like this should mean that nobody has any rights to your image, other then what is needed to view it while it is hosted on imgur. It seems to me that you could link to the image from a blog or something and spell out any particular rights or license that you wish to use for use beyond viewing on imgur.

You need to do the following:

• Click on Albums
• Click on the album you want add images to
• select Add or Remove images

A Select images for this album pop-up then appears. The boxes on the left allow to add images that are not in albums, or all images.
So click on Non-album images, select the ones you want moved into the selected album, then click Save.