Under current US law, and this is true for a lot of countries, copyright exists the moment you create a work, weither you put a copyright sticker on it or not. The label is a good reminder, but not nec. for the copyright to be enforce, unless you do certain works for hire.
This would or could include work for advertising companies, drug companies, and research under the aspes of a larger grant/lab or corporation.
Since your artwork does not fall under any of these conditions, you own it.
But it is helpful if you document your work so you can prove it if you have to. Dated digital pictures help.
It is not necessary for the date to be on the photo for this to work. The file has an encripted date imbeded in it when it is created. all files do.