So, the way the rights work is as follows:
As soon as an image is "fixed" (ie. completed) a copyright is created for that image, owned by the person who drew it. For somebody else to use it, publication rights must be secured (first electronic publication rights, first print publication rights, reprint rights, etc.). This is normally done through a written agreement (an agreement by email could hold up in court, but larger entities will usually do this with a proper signed and witnessed contract). What will sometimes happen is that an artist will complete a commission in what is called "work for hire" - this means that all of the copyrights are transferred to the client, allowing them to use it however they see fit.
(Work for hire was a pretty common practice in the comic books industry right up to the 1990s as far as I can tell, and there are a lot of horror stories that came out of it. A number of the independent comic book publishers founded in that time came out of a desire for artists to actually retain some ownership of their work.)
If you were to collaborate with somebody on a comic, there are two main ways you could handle the rights situation:
Arrange a work-for-hire contract with them. You would own all the rights to the art, but you would also probably have to pay the artist up-front for their work, so you need to be able to bankroll it.
Share the copyright of the comic itself with them, with the two of you splitting any proceeds in a negotiated ratio. This has a lot of benefits - they are more likely to be willing to work with you without demanding money up-front, and the collaboration itself will have add quality to your comic (I actually ended up changing the descriptions of a couple of characters in Re:Apotheosis when the cover art drafts ended up being better than what I had originally imaged for the characters - for example, Atria Silversword originally had long hair in my first conceptions of her). The drawback is that you need to spend time before the project starts making sure that you're likely to be on the same page and stay on the same page throughout - since you will only own half the copyright, your collaborator will have equal say in what happens to the comic (such as selling publication rights, adaptation rights, etc.).
I'd generally suggest going with the second option - you do have to share the copyright of the work, but so long as you've got a good collaborator, the benefits will far outweigh any drawbacks.