How to add copyright info to drawings?

I think all you would need to do is in Windows Explorer right click the file and under properties enter the copyright information.

You may want to look at the Creative Commons License. They may have suggestions on how to apply the license.

Perhaps this isn't what you need, but if you want to add a small line of text to an image, such as a copyright notice, here are two free image editors with that ability:
gimp.org/windows and getpaint.net (for Windows)
If you need to convert the image as jpg or png, here is a free tool that installs as a "printer" that can make an image or pdf file from nearly anything that you can print: pdfforge.org but I assume xnview.com is superior as an image conversion tool.