Can I use a painting of my own in an online article?

you can always use your own stuff, but remember the owners also own copyrights to the name, and other tings attached to he character, you can use your drawing of a mouse, but if you also call it Micky Mouse, you will be hearing from the Disney people about copyright infringement.

Actually, you can use a copyrighted image for the intended purpose as you’ve described above. (Be sure to point out that it is a copyrighted image, and who owns it. You don’t need to ask permission.)

This is the purpose of the Fair Use exception to copyright law.

My understanding is the same as that of @CWOTUS. And I think you are on much safer ground to use an image created and controlled by the maker than a copy that could be viewed as a misrepresentation. One of the big issues that I’ve heard of arising in legal cases is that the public may mistake an inferior (or simply unauthorized) version for the “real thing,” and that can be seen as harmful to the rightful owners.

Of course, as he says, you acknowledge the source and give full credit.

I would blur out the face like they do when protecting someone’s identity

I would think that cartoon images are copyrighted. If you paint your own Donald Duck, and call it a different name, the copyright would still be violated.