Can I store my iMovie files on iTune or iCloud without losing HD quality?

well, for one thing, you can't store movies on icloud.

You are going to need a third-party service if you want online storage of your movies or other files - iCloud does not provide general file storage. This page examines some options:

http://rfwilmut.net/missing3

This assumes you want storage, not actual online display of the movies. For displaying your videos, YouTube is widely known and used and is free, but converts your videos, reducing the quality in the process. Vimeo is less well-known but better quality: it's free for a basic account or $59.95 p.a. for more facilities and bandwidth.