That you bought them says to me someone else created them, whether of your product or not. A signed, dated, notarized letter from the creator of the items indicating you are the copyright holder would be beneficial, registering them (federal) would give you standing in a legal dispute. In fact, some cases may not even be heard without a registered copyright filing. Now, the good news is that for a really low cost, you can upload a digital copy of ALL OF IT at once, as much as your bandwidth will allow; you do not have to do them one by one.
Mailing it to yourself will work about as good as a note from mommy telling the nice officer it was ok for you to run 75 in a 20 zone, it's a myth and more likely to get you laughed at by a judge than anything. Last I looked it was like 35 bucks or so and you could upload thousands of items in one go, pretty damn cheap insurance.
Copyright exists the moment something is placed in tangible format, though there are some exceptions (you can't copyright an idea or concept, but you can copyright the drawing or description...) So a painting as soon as the brush hits canvas, music, the second it is recorded on a medium, etc. The person creating it is the copyright owner UNLESS it falls under work for hire, in which case there will typically (or should typically) be documentation/contract/etc. but again, register, hell, upload EVERYTHING you created that will fit in the allotted space/time.
I used to upload all my photos (hobby macro photography) in the same file with code and page layouts where I was listed in the contract as the owner, licensing the right to use to clients that didn't want to pay for ownership. There was room, it assigned to me, why the hell not!
