If you're actually concerned about copyright ownership, you need to do whatever is necessary in your country to legally register or establish it--you can't depend on having the raw file, because legally that means nothing. In the US, you're not even going to have a chance to present it in court, because unless you've registered it, you'll never get into the copyright court.
In the US, you can register hundreds of images online on one form for $35. Do that with everything you've shot every three months and you're covered.
Be sure you understand what copyright is. Copyright is legal "property," like having title to an automobile or a plot of land. When you create a work, as the author you automatically own it (possess the copyright). In the US, that's true even if someone else paid you to create it, unless you are a bona fide employee of that person (and taking pictures is actually part of your job) or you have expressly signed an agreement that you're "working for hire."
As the owner of the copyright to the work, you have total legal control over whether and how it can be used or copied (under certain limitatations--personal privacy rights and trademarks, notably). You normally license uses to other persons. Important: These are not "rights" these are "licensed uses." Make sure you've got the terminology correct.
If you actually intend to "transfer" copyright to someone else, you can do that as well. But this is almost never necessary and should carry a very high price tag. At the very most, you should be able to sell an unlimited license (the othe person can do absolutely anything he wants with the image forever) and still retain copyright for yourself. If you actually transfer the copyright to someone else, then you have no right yourself ever to use the image; in fact, in the US the other person can even claim he took the image himself (the US does not recognize "moral copyright").